CLIMATE CHANGE & THE UN COPENHAGEN SUMMIT

Br i efi ng Pa p er (09-05)
De c e m ber 2 0 0 9

CLIMATE CHANGE & THE UN COPENHAGEN SUMMIT
Summary

• Climate is changing rapidly now, and the evidence is that human activity in burning fossil fuels
for energy and deforestation are the primary causes.

• Computer models which have been successful in simulating the last 100 years of climate can be used to
produce forecasts of future change, based on different scenarios of future use of carbon-emitting fossil fuel. They suggest the potential for severe climate changes over the next 100 years with major impacts on human societies and their economies.

• Such forecasts inevitably have uncertainties relating to future carbon emissions and current scientific
understanding.

• Societies and governments have to decide whether to reduce greenhouse gas emissions now by
“decarbonising” their economies in anticipation of changes that might be smaller than forecast; or to wait and see, thereby risking that we will be too late and unprepared in responding to rapidly changing climate with great social, political and economic costs.

• Most governments, including those of the UK and Scotland, have in principle taken the former decision,
but none have yet taken the decisive action that will be required to meet targets that typically require emission reductions of the order of 60–80% by 2050.

• The Copenhagen Climate Summit will be vital in this regard. International agreement could in principle
create a “level playing field” that would reassure richer nations that their economies will not be disadvantaged compared with their economic competitors, and ensure that poorer countries are helped to adapt in “decarbonising” their economies.

• The direct effects of climate change to which Scotland will need to adapt may not be severe, and may
prove to be relatively readily manageable.The greatest effects on Scotland may however be the indirect economic impacts of economic and social disruption elsewhere, where the direct effects of climate change are much more severe.

• In order to achieve the targets that UK and Scottish Governments have set, it will be necessary to ensure
that the production and use of energy is largely “decarbonised” and that there are market and regulatory instruments that provide incentives for change.

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Purpose
1 increased melting of the Greenland Ice Sheet, and strong warming in the near-Arctic zones are a reflection of this.They are having major impacts on communities in those areas.Aqqaluk Lynge, President of the Inuit Circum-Polar Council, who was in Edinburgh in October, commented:“climate change is not just a theory to us in the Arctic, it is a stark and dangerous reality”. Although changes in Britain are less perceptible, they are evident and real. Works published by SEPA and SNIFFER1 show that in the last 40 years in Scotland, average winter precipitation has increased by almost 60% and in some areas of the west Highlands and the Hebrides it has doubled, whilst in the east the summers are drier. Days of frost and snow have decreased and the growing season has lengthened. 4 Scientific understanding is cumulative. The experimental approach of science is effective in identifying and discarding erroneous ideas. Its observations have reduced uncertainty in climate science and made the trajectory of global climate warming, and the evidence of human impact, clearer and more robust; even, ironically, at a time when it appears that public belief in these conclusions has waned.The changes that we now observe are consistent with model predictions made about a decade ago.The decade 2000 –2009 has been warmer on average than any in the preceding 150 years of instrumental observations. Precipitation in high latitudes has increased and decreased in the tropics, though sadly at the upper limit of model projections. The proportion of the Arctic Ocean covered by sea ice has continued to diminish, by about 30% between 1980 and 2009, with major implications for the Earth’s heat balance.Antarctic ice shelves are breaking up at a rate probably not seen since the end of the last Ice Age.All major glacier systems worldwide are retreating and global sea levels are rising, both at an accelerating rate. Emerging signals of change that are consistent with what we expect from model projections, but cannot yet be confirmed as systematic trends rather than inter-year variability, include: in the UK, heavier daily rainfall leading to local flooding; summer heat waves such as the summer of 2003 across the UK and Europe; globally increasing incidence of extreme weather events with unprecedented levels of damage to society and infrastructure. This year’s unusually destructive typhoon season in South East Asia, whilst not easy to attribute directly to climate change, illustrates the vulnerabilities to such events.There are persistent droughts, leading to pressures on water and food resources, and an increasing incidence of forest fires in regions where future projections indicate long term reductions in rainfall, such as South West Australia and the Mediterranean.

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The purpose of this briefing paper is to set out: scientific understanding of the nature and causes of the climate changes that are currently taking place; the status of forecasts of future changes and their impacts; the priorities for international collaboration in mitigating climate change and particularly for the United Nations Copenhagen Climate Summit in December 2009; and the priorities for Government in Scotland in meeting its carbon emissions targets and preparing to adapt to the climate changes that may in the near future be inevitable.

It is intended as a helpful summary for the Scottish Government and Parliament, for public and private bodies, and for fellow citizens. The scientific evidence for the nature and cause of change are presented in more detail in the accompanying science briefing.

Climate is changing rapidly NOW, and the evidence is that human activity is largely responsible
2 The global climate has shown a warming trend for the last 100 years and, averaged over the last 30 years, has changed at a rate that is faster than at any time since the end of the last Ice Age, 10,000 years ago.The evidence is that this latter change is primarily driven by the carbon dioxide produced by human activity that has been accumulating in the atmosphere since the Industrial Revolution and which acts as a “greenhouse gas”. Although there are many processes that influence the climate, such as variations in solar radiation, volcanic eruptions and “internal oscillations” within the climate system (see the science briefing), the evidence is that over the last 30 years, the effect of human-produced greenhouse gases has come to dominate over the other “natural” processes that in the past have determined the Earth’s climate.The assessment that humans are the primary driver of recent climate change takes all the other influences on climate into account, as well as variability generated within the climate system.The relatively slow rate of warming in the last few years is consistent with the expectation of an enhanced greenhouse effect super imposed on other “natural” determinants of climate variability. These climate changes are happening now. Previous scientific assessments predicted that impacts would be felt first in polar regions.The dramatic trend of reduction in the extent of arctic ocean sea ice,

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1 A Handbook of Climate Change Trends Across Scotland-Presenting Changes in the Climate Across Scotland Over the Last Century; SNIFFER; May 2006.The State of Scotland’s Environment; SEPA; 2006.

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Can we forecast the nature and rate of change of future climate?
5 The only basis for forecasting future trends is the extent to which we understand trends in the recent past.The crucial logic is that if we apply computer models of climate to the last 100 years of known climate, the models match the climate trends of the last 30 years only if the warming effect of human-produced greenhouse gases (GHGs) is taken into account (see science briefing).The conclusion drawn from this is that, over this period, the effect of human-produced greenhouse gases has progressively come to dominate over the natural processes that have normally determined variations in climate. If we then create scenarios of future greenhouse gas production (largely determined by our use of fossil fuels in energy generation), computer-modelled forecasts of future climate trends can be created. The most recent computer simulations 2, 3, 4, suggest that unless GHG emissions can be dramatically reduced, global average temperatures could increase by up to 60C in the next 100 years, with 2–40C as the most likely range.This is equivalent to the amount of warming that has occurred from the coldest part of the last Ice Age, when Edinburgh and Glasgow were covered by a 1km-thick ice sheet, to the present day. In the Arctic, where glaciers are already melting strongly, warming of up to 160C is a strong possibility. The transfer of water to the oceans from melting glaciers coupled with thermal expansion of the oceans could result, in worst case, of a global sea level rise of up to 2m by 2100. Simulations also suggest a dramatic reorganisation of the pattern of global rainfall, with the probability of increases in high latitudes and reductions in mid- to low-latitudes, where much agriculture already suffers from low rainfall and arid conditions. Increases in high latitude precipitation are already detectable. How confident can we be about these forecasts? There are many uncertainties: we do not know the future of GHG emissions; we do not yet understand some natural processes sufficiently to include them in the climate models; and there may be processes that we do not yet know about which could have a major effect on climate. Accepting the uncertainties, current scientific understanding, shared by almost all practising climate scientists, is that if the current trend of emissions is continued, there will be a general acceleration of the rate of global climate change,
2 Meteorological Office, Hadley Centre, September 2009. 3 Climate Change 2009: Science Compendium. United Nations Environment Programme, September 2009. 4 Statement of Dr R.K. Pachauri, UN Summit on Climate Change, 22 September 2009

with fluctuations that reflect the operation of other influences on the climate system. A further uncertainty is that although the computer simulations show a gradually accelerating rate of change of global climate, we cannot rule out sharp, step changes and associated shifts in the way the global climate organises itself.

How should we respond in the face of these uncertainties?
8 The above represents the current assessment of climate science about recent and potential future changes.The answer to the question:“what, if anything, should be done?”, lies outwith the scientific domain. It is a matter for citizens and for political debate, but a debate that needs a more realistic view of science.There is a popular but erroneous view that science delivers certainties. It does not, and cannot.This applies equally to most of the sciencebased technologies on which modern society depends. Not only do we individually cope with uncertainty in our daily lives, but uncertainties remain in the every-day technologies that most of us use. But the potential for climate change is politically more difficult to deal with.This is partly because of inherent uncertainties, partly because many vocally reject the thesis of human-induced climate change (though very few are practising climate scientists), because mitigating change and preparing to adapt to its potential severity require massive changes in our economy and lifestyle, and because of the immense difficulty of achieving the unprecedented levels of international cooperation that will be required for effective global action. The dilemma is whether to reduce GHG emissions now by “decarbonising” the global economy in anticipation of changes that might not happen, or to wait and see, thereby risking that we will be too late and unprepared in responding to rapidly changing climate with its enormous social, political and economic costs. In the event, the consensus of the international community, and increasingly of international business, has been to take the former approach, whilst many individual countries are setting out to achieve very demanding carbon reduction targets.The UK and Scotland are amongst the leaders in the latter. It will nonetheless be crucial, if governments are to take the difficult steps required to achieve their targets, that citizens accept that the above trade-off, in the face of uncertainty, is the wisest choice.

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The magnitude of potential impacts
10 The rationality of such a choice depends upon the severity of the impacts that human-induced climate change could have, and that we would wish to avoid. The direct impacts of climate change in Scotland may be relatively slight compared with other parts of the world, and relatively easily manageable, the principal potential impacts being: wetter winters; warmer summers; shifts in the biosphere with impacts on agriculture and wildlife; new patterns of disease; potential increases in the magnitude and frequency of river valley, urban and coastal flooding, with implications for the resilience of national infrastructure. (There remains however the possibility of a major reorganisation of Atlantic Ocean circulation, which could have a greater impact on Scotland). In themselves, these may not seem to justify the massive and potentially expensive shift to a decarbonised economy that is required if Scotland is to meet its legally-binding emissions targets.The argument is that as one of the prime beneficiaries of the fossil fuel economy of the last 200 years, we bear an ethical responsibility to those in other parts of the world that have not benefited to the same degree and yet are likely to suffer the most extreme environmental consequences of climate change. 11 The most serious consequences for Scotland are likely to be indirect.Adverse climate change multiplies the complexity of international relations in an already turbulent world. It would compound existing bottlenecks and supply constraints in food, energy, water and in other critical natural resources and infrastructures, creating new and hard-to-manage political and economic instabilities. Climate-related risks act to exacerbate a range of existing socioeconomic pressures created by rapid industrialisation, population growth, urbanisation and economic growth, and also increase the risk of conflict. Thus, in addition to the potentially minor direct effects of climate change on Scotland, these indirect effects, with their potential to disrupt the global economy, may prove to be much greater risks.They add another dimension of self-interested rationale for Scotland’s climate policies in addition to the ethical imperative 5. Doubling or tripling of food and energy prices as a consequence of events elsewhere would be an uncomfortable reality. 12 The probability that impacts elsewhere will have major indirect consequences for Scotland can be assessed by exploring the continuation of well-established present trends and by simulations of future climate. By 2020, yields from rain-fed agriculture (the dominant method) in some African countries could be reduced by up to 50%. The global trend of decay of glaciers and snow cover is already reducing melt water from major mountain ranges. Glacier melting in the Himalayan & associated mountain chains currently supplies most of the dry-season flow of the great rivers of Asia (Indus, Ganges, Brahmaputra, Mekong,Yangtze,Yellow) on which 20% of humanity depends for much food and energy production. It is estimated that continuation of current trends could reduce dry-season flow of those rivers by more than 60% by 2050. More than 20 million people were displaced by sudden climate-related disasters in 2008 alone.An estimated 200 million people could be displaced as a result of climate impacts by 2050 6.

The international setting: mitigation and adaptation
13 The first international efforts to reduce GHG emissions in order to limit the extent of human-induced global climate change led to the Kyoto Protocol, signed in 1997 and ratified by 187 countries by 2009. 37 industrialised countries agreed to reduce their GHG emissions by 5.2% on average compared with 1990 emissions.The USA, responsible for 36% of 1990 emissions, did not ratify.The Kyoto Protocol has been a failure. It has had no impact on global greenhouse gas concentrations, which have continued to rise. Indeed the rate of emission has almost doubled since the year 2000. 14 There has been a debate whether to concentrate on mitigation of climate change to the exclusion of discussion of adaptation to change, because discussion of the latter might weaken efforts for the former. However, computer simulations suggest that even in the unlikely event that global human emissions began to fall before 2020, global temperatures would continue to rise because the slowly warming oceans would continue to be a source of a net flow of carbon dioxide into the atmosphere.There would remain a significant probability that global temperatures would rise by 20C, which has been regarded as a threshold for powerful global impacts that would seriously affect societies and economies

5 Having been amongst those countries that have benefited most from fossil carbon fuels, we are also amongst those that have contributed most to human-induced climate change, for which those in other, poorer countries will pay most of the price. 6 UN, IPCC, Stern Review 2006

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15 Under these circumstances, many governments, including those of the UK and Scotland, now not only place emphasis on mitigating change by reducing GHG emissions, but are also developing national policies to adapt to the changes that may be inevitable within their own territories. 18 The two largest emitters of greenhouse gases, the USA and China, are key participants. The Obama administration in the USA, which has hitherto been reluctant to specify emission reduction targets, has now offered to cut emissions by 17% from 2005 levels by 2020, 42% by 2030, and 83% by 2050. In contrast, the EU has set a 2020 target of 20% reduction from a 1990 baseline, to be increased to 30% in the event of global agreement. Using the same 1990 baseline, the US 2020 target is only a 4% reduction, and even this must pass the hurdle of US Congressional ratification, which is highly uncertain. 19 China has so far only committed itself to reducing the “carbon intensity” of its economy by 40–45% by 2020 compared with 2005 levels.This means a 40–45% reduction in the amount of carbon used per yuan of production value, and ties emissions reductions to GDP. On this basis, and because of its high rate of economic growth, China’s emissions are likely to continue rising rapidly for at least ten years. It seems very likely that USA and Chinese positions are strongly linked: that the willingness of each one to act more decisively will depend on the willingness of the other to do the same. 20 On 28th November, 53 Commonwealth Heads of State backed the British Prime Minister’s call for a 10-billion-dollar Copenhagen Launch Fund, to help the poorest countries invest in carbon emission controls and to adapt to a warming climate. This has been supported by some non-Commonwealth representatives, including French President Nicolas Sarkozy and Danish Prime Minister Lars Lokke Rasmussen. 21 A variety of economic instruments have been suggested to drive the economic and technological changes that will be required to reduce emissions. They include putting a price on carbon, carbon taxes, and so called ‘cap-and-trade’ mechanisms, which governments specify a cap on the amount of carbon that can be emitted and allocate or auction emissions permits which themselves can be traded in a carbon market. In principle these mechanisms should stimulate the adoption of low carbon technologies by business and research into new technologies. But it will also be vital for funds and technologies to flow to poorer countries to permit them to adapt without impoverishment.

The United Nations Climate Change Conference in Copenhagen
16 The potential for global collaboration to address the reality of current change and threat of imminent future change now centres on the inter-governmental conference taking place in Copenhagen from 7th–18th December 2009.The most direct political route to mitigate the threat of climate change would be for the Conference to agree legally-binding targets for global CO2 emissions to peak between 2015 and 2020, and to be reduced to about 1 tonne per capita per year by 2050.This compares with current global CO2 emissions of about 4.5 tonnes per capita per year. The problem is that OECD countries emit on average about 11 tonnes per capita per year on a direct production basis, whilst amongst fast developing countries, India emits 1.1 and China about 4.6 tonnes per capita per year, the USA averages 19.8 tonnes per capita per year, whilst the average Kenyan manages on 0.3 tonnes per capita per year7.Thus, although China has surpassed the USA as the world’s largest national emitter, its per capita emissions remain much lower than those of the USA. Moreover, much of China’s emissions can be accounted for by the manufactures that China produces for other countries.The dilemma is that all groups recognise the potential impact on their economies of the severe reductions in GHG emissions that are required, whilst also recognising the potentially greater impacts of failing to reduce global emissions. 17 There is broad acceptance of the principle that rich, developed countries should shoulder a proportionately larger burden of near–term reductions, that economically developing countries should continue to increase their energy use, whilst reducing its carbon-intensiveness, and that relatively poor countries should receive financial support in decarbonising their economies.This is the so-called “contract-and-converge” principle. The political task remains enormous however, and whilst we must hope that the Copenhagen meeting will make legally-binding agreements, as a minimum it should be the beginning of an urgent process to produce a global agreement that is robust enough to address the problem.

7 Energy Information Administration; International Energy Annual 2006;World per capita CO2 emissions from the consumption and flaring of fossil fuels 1980-2006

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The UK and Scotland
22 In 2007, the UK emitted about 9.7 tonnes and Scotland about 8.2 tonnes of carbon per capita per year on a direct production basis. The UK Government has set a target for carbon emissions reduction compared with 1990 of 34% by 2020 and the Scottish Government of 42% by the same date. Both have set targets of 80% reduction by 2050. 23 In order to achieve these targets, it will be necessary to ensure that the production and use of energy is decarbonised and that there are market and regulatory instruments that provide incentives for change. In energy generation, emission reductions that also minimise cost and maximise security are most likely to be achieved if generation companies are free to use a wide range of technologies, if the transmission and supply network is extended and able to take efficient generation from wherever it is available, and if there is a price or tax on carbon that benefits low carbon generators.The Scottish Government’s decision to withhold planning permission for new nuclear stations reduces its options, so that satisfying base load demand will be dependent on the hope that carbon capture and storage (CCS) facilities can be funded and implemented to reduce emissions from coal-fired power stations, whilst generating electricity at competitive prices. Improvement of the transmission network between Scotland and England is a priority, partly to maximise efficiency, partly to permit export of Scottish renewable energy, and partly to provide insurance in case the hoped-for CCS developments do not materialise. 24 There are two key priorities for energy use. First in reducing emissions from buildings through improved heat insulation and increase of renewable heat sources, and secondly in reducing transport emissions (currently the fastest growing source) by improving fuel efficiency, shifting to electric propulsion and better transport planning. 25 In relation to drivers for change, it is important that reductions in emissions that occur as a consequence of recession are not used to reduce targets for succeeding years.The dramatic price reduction in the EU emissions trading scheme in recent months will reduce incentives for the investments needed for low carbon technologies. It is vital that government considers new rules to strengthen the investment climate for low-carbon power generation, to mitigate the risk that investment will continue to flow predominantly to conventional fossil fuel generation. 26 Both the Scottish and UK Governments have been developing strategies for adapting to climate change. This should involve not only adapting to the direct consequences of change and its costs. It will also be necessary to consider indirect impacts of climate change: those that arise because of economic and social disruption elsewhere and which Scotland or the UK would not be able to avoid. At the same time there are opportunities for Scotland to use its research and commercial strengths to exploit the global drive towards a low-carbon economy, thereby minimising any negative impact on its own economy.The Royal Society of Edinburgh has begun a major inquiry, entitled “Facing up to Climate Change”8 to address these issues, which, it is hoped will be completed early in 2011.

8 http://www.rse.org.uk/enquiries/climate_change/index.htm

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CLIMATE CHANGE & THE UN COPENHAGEN SUMMIT THE SCIENTIFIC EVIDENCE
Introduction
1 This paper is intended as an accompaniment to the Royal Society of Edinburgh’s Briefing Paper on climate change and the UN Copenhagen Summit for those who wish to know more about current scientific understanding about the state and potential future of the Earth’s climate, on which the discussions at Copenhagen are based.

The natural variation of climate
2 The Earth’s climate has shown great variability in the past. In the last million years of geological time, the Earth has undergone a series of Ice Ages (Figure 1), when relatively short, warm periods, such as the present, have separated longer, cold, glacial periods, when the middle latitudes of Eurasia and North America, including Scotland, have been covered by ice sheets several kilometres in thickness, similar to those of modern-day Greenland and Antarctica. Research in the 1960s on sediment cores from the deep oceans, which contain biological and geochemical archives of past variations in global climate, showed that the tempo of climate variation in the Ice Ages correlates extremely well with changes in the Earth’s orbit round the Sun, and thus that the resultant variations in the magnitude and variation of solar radiation reaching the Earth have controlled these large scale patterns of climate change1.Work on former climates from many parts of the world permitted estimates to be made that indicated a difference in global average temperature of about 50C between the cold, glacial periods and the warmer intervening periods, although locally, as in NW Europe, the difference was much larger. However, it was also calculated that orbital changes could only produce maximum variations in global mean temperature of about 0.50C, one tenth of that indicated by the geological record. It was therefore concluded that although orbital changes were clearly the “pacemaker” of the Ice Ages, some other process must be responsible for amplifying the solar orbital signal.

(a)
CO2 ppmv Degrees C

Figure 1 Climate fluctuations during the last 400,000 years of the Ice Ages: (a) This shows the record of temperature (in blue) and carbon dioxide (in red). The cold, glacial periods are typically about 100,000 years in length, and the intervening warm periods about 10,000 years. (EPICA Programme); (b) This shows the carbon dioxide record, including the very recent strong rise in carbon dioxide concentrations to above anything we have known in the recent geological past, and the possible future change if emissions continue on the present trajectory. (C. D. Keeling and T. P. Whorf; Etheridge et.al.; Barnola et.al.; (PAGES / IGBP); IPCC)

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CO2 Concentration (ppmv)

Years Before Present

1 Hays, J.D et al. 1976.Variations in the Earth’s Orbit: Pacemaker of the Ice Ages. Science, 194 (4270), 1121.

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The last glacial period ended about 10,000 years ago. Since then although the magnitude of climatic variation has been much smaller, it has been significant. It shows variability on timescales shorter than those driven by orbital changes.A number of processes are deduced to have been responsible for them. Some changes have been driven by so-called “external forcing”, when processes outside the climate system drive change, such as short-term variations in the strength of solar radiation, or when major volcanic eruptions inject a dust cloud into the stratosphere that shields the surface from solar radiation and thereby cools global climate. Other, more complex changes occur as a consequence of so-called “internal oscillations” that take place within the climate system itself.A well-known example of this is the “El Niño” effect. In the tropical Pacific, easterly winds normally push warm surface waters, and the rainfall associated with them, towards the west, towards Australia and New Guinea. However, these winds tend to weaken for a few weeks around Christmas time (El Niño refers to the Christ child), so that warm surface waters, and the associated rainfall, extend towards South America. Every 4–7 years, El Niño lasts for many months, with severe climatic and economic knock-on effects world-wide.The most severe in recent years was in 1997–98. Back in the late 1960s and early ‘70s, it was naturally assumed that the pattern of climate change shown in Figure 1a would mean that the Earth would soon return to glacial conditions, and many scientists suggested that this would be the near-future of climate. However, by this time, scientists were also becoming aware of the measurements from Hawaii, started in 1958, that showed annual increases in atmospheric carbon dioxide concentration (Figure 2).They realised that this increase must largely be a product of increasing emissions from human activity, particularly the burning of fossil fuel (coal, oil, gas) for energy production.They also speculated that if the trend were to continue, the well-established greenhouse effect would produce an increase in global temperature rather than a descent into another glacial period. Later evidence from the ice sheets showed how this human-produced increase had taken carbon dioxide levels far above any known in recent geological history (Figure 1).

The role of greenhouse gases and the link between human activity and climate change
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Figure 2 The record of atmospheric carbon dioxide concentration from Hawaii, also showing the annual fluctuations. The record reflects global changes because the global atmosphere mixes rapidly, over a period of 1–2 months. Note how concentrations continue to rise after the 1997 signing of the Kyoto Protocol. (Data from: Dr. Pieter Tans, NOAA/ESRL www.esrl.noaa.gov/gmd/ccgg/trends).

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It was also realised that variations in the atmospheric concentration of carbon dioxide could be the means of amplifying the solar orbital signal received from the orbital “pacemaker” to achieve the magnitude of global temperature fluctuation characteristic of the Ice Ages.The process would be: increases in solar radiation warm the Earth – the oceans warm slowly, and as they do, there is a net release of carbon dioxide – this warms the atmosphere due to the greenhouse effect, and so on.A characteristic of such a process is the “feedback loop”, whereby temperature increases would lead the rise of carbon dioxide in the atmosphere, which would then intensify the warming effect.This lead–lag relationship has now been observed in cores from the Antarctic ice sheet. The so-called greenhouse effect was first analysed quantitatively by the Swedish chemist Arrhenius in 18962. The Earth and other planets receive solar radiation in a wide range of wavelengths. Some of this is radiated back from the planetary surface in the infra-red frequencies, and certain gases (so called greenhouse gases – GHG), principally water vapour, carbon dioxide, methane and nitrous oxide, absorb much of this radiation, and re-emit it in random directions. Some of this heat is radiated to space, some to the Earth.This process is not a complex one, but a simple physical property of the greenhouse gas molecule. It is a process that conditions the surface temperature of all the planets of the solar system. Increase the GHG concentration, and planetary surface temperatures are higher, reduce concentrations and temperatures are lower. If the Earth were to lose all its atmospheric gases, its surface temperature would be reduced by about 320C, and life as we know it would cease. It was because of this that predictions of a new glacial period were replaced in the early 1970s by predictions of global warming. Carbon dioxide poses a particular problem, because the long average residence time in the atmosphere of a molecule of carbon dioxide is about 100 years. From the beginning of the Industrial Revolution until 2004, about 1160 gigatonnes3 of carbon dioxide have been released into the atmosphere from fossil fuel burning and cement production. If all this carbon dioxide had stayed in the atmosphere, concentrations would have risen by 160 parts per million (ppm) since pre-industrial times.The actual rise has been about 100ppm. So roughly 60% of what has been emitted has accumulated in the atmosphere.The rest has been absorbed in the ocean or sequestered by growing trees. It means that if we stopped emitting carbon dioxide now, atmospheric levels would remain anomalously high for hundreds of years. It is also important to note that we are “short-circuiting” the natural system.Whereas in the past, there has been a natural lag between warming of the atmosphere and warming of the oceans, resulting in a delay between initial warming and carbon dioxide release from the oceans, we are now injecting carbon dioxide directly into the atmosphere, thus short-circuiting the natural delaying processes of the ocean. Several independent estimations have now been made of the global or hemispheric average temperatures for the last two millennia. Figure 3 is one of these, and shows that the late 20th Century warming has been rapid and large compared with earlier periods (note that this is independent of the University of East Anglia reconstruction, about which there has recently been much controversy). If we look in more detail at the 20th Century warming however (Figure 3), we see that the pattern of climate change has been much more complex than the smoothly accelerating pattern of greenhouse gas concentration (Figure 2).We expect the record of climate change to be complex, partly because climate is influenced by a series of separate causes whose individual impacts will vary through time, and partly because of the complexity of the climate response, as we see with El Niño events. For example, the warmest year on record was 1998 (Figure 4a), leading some who doubt the reality of human-induced climate change to seize on this as evidence that global temperature is unconnected to the continuing increase of GHG concentrations. However, even if GHGs are strong drivers of climate change, we would not expect climate to follow precisely the trend of GHG increase. 1998 was also the strongest El Niño event on record, which naturally gave a strong temperature spike.The reality is that eight of the last ten years have been amongst the warmest on record, even though the intensity of radiation from the sun has been diminishing since about 2000.

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2 Charney J. G. et al. 1979. Carbon Dioxide and Climate: a Scientific Assessment. US National Academy of Sciences, Climate Research Board. 3 1 Gigatonne = 1 Billion tonnes.The emission statistics are from Marland, G. et al, Global regional and national CO2 emissions. In Trends:A compendium of data on global change. US Dept of Energy. 2007.

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Figure 3 Estimates of mean decadal temperatures over the land areas between 300 and 900 in the northern hemisphere during the last 1500 years. Prior to the instrumental record of the last 150 years (shown in red), temperatures are deduced from tree-rings, lake sediments and ice cores. The dashed lines show the range of higher frequency variability in the data. The record shows an early mediaeval cool period from prior to about 950AD, a mediaeval warm period until about 1200AD, the so-called Little Ice Age from about 1450 to 1850AD and the very strong late 20th Century warming. Temperatures in sub-surface rocks can be used to deduce long-term changes in surface temperature that naturally smooth out inter-annual variations to show long-term trends. Temperature records from 631 boreholes have been used in this way to show how distinctive the 20th Century warming has been compared with the preceding 400 years.
(From: Hegerl, G.C. and others. 2007. Detection of human influences on a new, validated 1500-year temperature reconstruction; Journal of Climate, 20 (4): 650-666.)

(a)

Figure 4 Recent trends in global average temperature: (a) The black line shows the measured global mean. The green line shows the forecast future based on models. The peak global mean temperature in 1998 coincides with the largest observed El Niño event. The period from 1998 to the present, globally cooler than 1998, but warmer than years preceding 1998, coincides with a period of diminishing solar output since about 2000, which may have restrained warming. Some have claimed that the climate has been cooling since 1998 (e.g. Investor’s Business Daily, 2008). The longer-term trend is one of warming, with 1998 being a year of exceptionally strong (El Niño reinforced) warming. The cooling of 1992 was probably the influence of the Mount Pinatubo volcanic eruption. The diagram illustrates how different processes influence global climate, with the postulated underlying trend due to increasing GHG concentrations; (b) Global average temperatures since 1850, showing the irregular nature of the climate record. (UK Met Office).

© Crown Copyright 2009, the Met Office

(b)

© Crown Copyright 2009, the Met Office

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However, just because there is a correlation between two variables, this does not demonstrate cause and effect. The crucial argument that GHG emissions have come to dominate amongst the causes of climate change over the last 30 years comes from computer modelling. The term is an unfortunate one, as computer models are not models in any trivial sense, but means whereby our most rigorous understanding of the physical, chemical and biological processes that influence climate, represented in mathematical form, are analysed.The critical computational experiments have been to simulate the last 100 years of global temperature change. In one series of model runs (Figure 5b), the causal drive is provided by the natural factors which we believe influence climate: the Earth’s rotation, solar radiation, volcanic eruptions etc, but with atmospheric GHG concentrations typical of the pre-industrial era.This model produces a very good agreement with reality for the period up to about 1970, but after that, the model suggests a cooler Earth than really occurred. In another series of experiments (Figure 5a), the observed GHG increases were included.These not only successfully simulated the pre-1970 climate, but also the post-1970 period.The strong conclusion from these experiments is that after about 1970, man-made GHG increases have begun to dominate over other causes of climate change.

(a)

Figure 5 Results of computer modeling experiments: (a) The heavy black line shows the trend of global average temperatures over the last 100 years. The red line shows the results for a model in which both natural forcing and forcing from human-produced greenhouse gases are included; (b) Results of an experiment in which human-produced greenhouse gases are omitted. The black line again shows the global average temperatures and the blue line the model results. The conclusion is that since about 1970, human-produced greenhouse gases have had a greater influence on the changes in average global temperature than “natural” processes, and that without them, the global average temperature would have been significantly lower. The vertical lines show major volcanic eruptions during the period. (Hegerl, G.C., F. W. Zwiers, et al, 2007: Understanding and Attributing Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IPCC).

(b)

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The future of climate
10 In principle, because we can calculate the probable future of solar emissions and other cyclical processes, and if we could know the future of human GHG emissions, we could also forecast the future trajectory of global climate. This also assumes that there are no important processes that we do not know about or do not understand that would undermine the validity of forecasts. It also assumes that such relatively unpredictable events as volcanic eruptions would be like the short-term eruptions that we have known in recent millennia, which have only had a short-term climatic effect, and not major, sustained super-eruptions that have occurred in the geological past. With these latter provisos, we should be able to estimate the future of climate in relation to the future of human GHG emissions. 11 Figure 6 shows forecasts of future global temperatures based on different GHG emissions scenarios and using models similar to those that were used in the experiments shown in Figure 5. It is particularly important to note that even if emissions were to drop dramatically in the near future, GHG-driven climate change would continue, for two reasons. First, the long residence time of carbon dioxide in the atmosphere would mean that GHG concentrations would fall only very slowly; and secondly, because the oceans are still warming in response even to present temperatures, they will lose more carbon dioxide to the atmosphere as they warm further, which will in turn continue the warming effect. It is estimated that the slow warming of the oceans will continue for several hundred years, and that we may therefore need to continue efforts of climate control and adaptation long into the future, irrespective of what happens in the short term.

(a)

Figure 6 Scenarios for the future: (a) This shows possible future scenarios for carbon emissions. Note that the current trajectory of increasing carbon emissions lies along the steepest line; (b) The results of global climate models that forecast future global temperature changes. The colours match those of the emissions scenarios in (a). Note that even the steepest decline in carbon emissions in (a) is forecast to produce a global temperature rise of 20C, regarded by many as the threshold for “dangerous climate change”. (Reproduced from a presentation given by Professor Julia Slingo, Met Office Chief Scientist, at the RSE on 22nd October 2009 http://www.rse.org.uk/enquiries/ climate_change/talks_slides/ j_slingo_slides.pdf)

(b)

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12 Future scenarios of change and impact depend fundamentally on the “carbon cycle”, which refers to the way in which carbon behaves in nature.There are “sources” of carbon, where carbon is released into the atmosphere, and “sinks” of carbon, where it is lost from the atmosphere.The atmospheric concentration of carbon dioxide depends on the balance between these sources and sinks.The great reservoirs of carbon, which can either act as sources or sinks are: within the solid Earth, within the oceans, and within the surface biosphere (forests, peatlands): The solid Earth. This is a major net source from which we extract the fossil hydrocarbons that we exploit for energy, and from which we release 8.4 GtC/yr (gigatonnes of carbon per year) for energy fuel from accessible reservoirs estimated to contain about 1600 GtC.There is also a large amount of carbon locked up in solid methane hydrates beneath the world’s continental shelves.A possible, though improbable, impact of a warming ocean could be to release some of this reservoir of carbon. It has happened before in Earth history, with massive environmental consequences. The oceans. The ocean reservoir contains about 40,000 GtC.There is a net flow of carbon dioxide from the atmosphere to the oceans amounting to about 2 GtC/yr, although some recent research shows that this rate of carbon uptake in ocean surface waters may be decreasing.The excess of carbon accumulating in the oceans is deposited on the ocean floor as lime-rich sediment.A warming ocean may reduce the net uptake of carbon, an effect that could be intensified by acidification of surface waters, because of the reduction in the concentration of ocean plant life that absorbs carbon dioxide. The biosphere. Soils and vegetation together are a reservoir for about 3700 GtC.There is a net uptake by them of about 1.5 GtC/yr, but these flows are very difficult to measure, particularly for forest. Whilst the continuing loss of forest cover reduces the area of the forest sink for carbon, the increase in atmospheric carbon dioxide, which acts as a fertiliser for plant growth, may increase the take-up of carbon by individual trees.The fear is that increasing aridity, forecast by climate models for many areas of tropical forest, may then reduce the carbon take-up by individual trees, possibly turning some major forest areas into net sources of atmospheric carbon.The carbon balance determined by the interaction of these sources and sinks is therefore a net flow into the atmosphere of between 4.9 and 6.4 GtC/yr, the uncertainty being derived from the uncertainty of biosphere take-up. This net flux to the atmosphere determines the atmospheric concentration of carbon, which we know, from the Hawaii measurements (Figure 2) to be increasing year by year. Predicting the future of the carbon balance is yet another uncertainty in our capacity to forecast future change. 13 The impacts on human society of potential future climates should clearly be the determinant of any targets for emissions reduction. Compared with the uncertainties inherent in future climate forecasts, there is greater clarity and reduced uncertainty about the impacts that specific climate changes would have across a wide range of systems, sectors and societies.Whilst a global temperature increase of up to 10C may be beneficial for a few regions and sectors, such as high latitude areas and agriculture, most surveys suggest increasing damage if the Earth warms to between 10C and 30C above current levels. Serious risk of large scale, irreversible system disruption, such as reversal of the land carbon sink (rapid decay of major forested areas because of increasing aridity) and possible destabilisation of the Greenland and Antarctic ice sheets is likely above 30C. Such levels are well within the range of climate change projections for the century.We are now also measuring significant increases in the acidity of the oceans due to absorption of carbon dioxide, which could reduce their capacity to absorb more carbon dioxide from the atmosphere and affect the entire marine food chain.Whilst it is recognised that the precise level of global temperature change at which economic and social impacts become very severe is difficult to define, a consensus has arisen that targets for emissions reduction should be set that keep the global temperature rise to below a 20C threshold.

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Additional Information and References
The following are Royal Society of Edinburgh reports relevant to this topic:

• • • • •

The Royal Society of Edinburgh’s submission to the Scottish Parliament Transport, Infrastructure and Climate Change Committee’s consideration of the Climate Change (Scotland) Bill (February 2009) The Royal Society of Edinburgh’s submission to the Scottish Government’s consultation on Scotland’s Climate Change Adaptation Framework, Preparing for a Changing Climate (June 2009) The Royal Society of Edinburgh’s input to the 3rd Stage Debate in the Scottish Parliament on the Climate Change (Scotland) Bill (June 2009) The Royal Society of Edinburgh’s submission to the UK Department of Energy and Climate Change, A Framework for the Development of Clean Coal (September 2009) The Royal Society of Edinburgh’s paper on the Development and Deployment of Carbon Capture and Storage in Scotland (September 2009)

Any enquiries about this response and others should be addressed to the RSE’s Consultations Officer,William Hardie (Email: [email protected]) Responses are published on the RSE website (www.royalsoced.org.uk).

The Royal Society of Edinburgh (RSE) is Scotland’s National Academy. It is an independent body with a multidisciplinary fellowship of men and women of international standing which makes it uniquely placed to offer informed, independent comment on matters of national interest. The Royal Society of Edinburgh, Scotland's National Academy, is Scottish Charity No. SC000470

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