Sir David King ScD FRS, Chief Scientific Adviser and Head of the Office of Science and Technology, 1 Victoria Street, London SW1 0ET, UK.
Globally we face serious challenges from the effects of climate change. The causal link between global warming and increased greenhouse gas emissions is well established. Carbon dioxide levels are at a higher level than at any time in the past 750 000 years at least, and it is too late to stop further warming and consequent impacts on UK and global societies. Here I summarize the latest scientific evidence for anthropogenic global warming and outline strategies for adapting to its impacts and mitigating the effects in the longer term.
The science of climate change is a mature subject. It was the French mathematician Fourier in 1827 who put forward the greenhouse gas concept: our atmosphere absorbs heat that would otherwise radiate out into space. The greenhouse effect has two important consequences for Earth. First, the Earth's average surface temperature is kept at about 15 °C by this blanket effect of the atmosphere that surrounds it. Without the greenhouse effect, life on this planet would not exist as we know it: in the absence of the warming effect of greenhouse gases, the average surface temperature would be −18 °C. Secondly, night-time temperatures would be much lower than they are without the blanketing effect provided by the greenhouse gases.
Fourier's work was followed in 1860 by the discovery by the British scientist Tyndall that the major constituents of our atmosphere, nitrogen and oxygen, do not absorb heat (infra-red radiation from the Earth) and that the greenhouse effect is the result of the minority gases in our atmosphere, especially water vapour, carbon dioxide and methane. These are termed the greenhouse gases. On this basis the Swedish Nobel prize winner Arrhenius carried out the first global warming calculation in 1896. He said: ‘If the human population should burn so much fossil fuel that the carbon dioxide level in the atmosphere should double, what would the temperature rise be?’ His calculation yielded an average global increase of 5 °C. This is a very large increase: to put this in context, the difference in temperature between an ice age and a warm period is about 5–8 °C.
More recent calculations of the global temperature increase that results from a doubling of carbon dioxide levels have yielded a temperature range of 1·5–4·5 °C, and research from the Hadley Centre proposed a temperature range for doubled carbon dioxide of 2·4–5·4 °C, but these results, and other recent research, do not rule out the possibility of even greater temperature change, suggesting that the scale of climate change could be quite different from previous predictions.
Carbon dioxide levels in the atmosphere, and water isotopes serving as a proxy for global temperatures, are encapsulated in ice layers in the Antarctic and on Greenland. The latest data from ice cores (McManus 2004; Augustin et al. 2004) show that over the past 750 000 years the Earth has been through eight cycles of glacial and interglacial periods. In each of the ice ages the carbon dioxide levels were at around 210 parts per million (p.p.m.) in the atmosphere. In each of the warm periods the carbon dioxide levels rose to 260–280 p.p.m. Temperature changes were 5–8 °C. The correlation bears out the greenhouse model described above.
Carbon dioxide levels in the atmosphere have been accurately recorded at the Mauna Loa Observatory, Hilo, Hawaii, since 1959. They are now rising at a rate of close to 2 p.p.m. per annum, reaching 379 p.p.m. in 2004, 40% above the pre-industrial levels of our current warm period and also all previous warm periods. Global temperatures have increased by 0·6 °C over the past century. This is in broad agreement with the theory of the greenhouse effect, and is what Arrhenius predicted in 1896.
Some, not within the scientific community, argue that warming is happening but that it is simply part of a natural cycle. They point to the relative warm period in Europe in the Middle Ages and the cooler conditions that occurred in the 17th and 18th centuries, the so-called mini-ice age. There are undoubtedly natural fluctuations but we are now facing a sustained warming trend that can only be explained by the rise of greenhouse gases in the atmosphere. The effect of such warming is seen in glaciers, which are in retreat around the globe. Some have been dated back to the last ice age, 12 000 years ago, and are in retreat for the first time in our warm period. The latest supporting data were published from the Chinese Academy of Sciences late last year (Tandong et al. 2005). A 25-year analysis of the Chinese glaciers, which correspond to 15% of land-based ice on Earth, based on more than 30 000 aerial photographs and satellite images, has demonstrated the loss of 8000 km2 of ice cover in this period. Tandong et al. (2004) estimate that the Chinese glaciers will have disappeared by the end of the century.
Impacts of climate change predicted by the modellers are already occurring. Climatologists are currently focusing their attention on a number of key factors.
what are the extreme events that we are vulnerable to as global warming continues?
These extreme events include a slowing, or even termination, of the thermohaline circulation that maintains northern temperatures about 8 °C higher that they would otherwise be (Wood et al. 2005; Challenor, Hanlin & Marsh 2005); loss of the Greenland ice sheet, which would produce global sea level rises of about 7 m over a time-scale of a thousand years or more (Gregory, Huybrechts & Raper 2004); enhanced retreat of glaciers in some regions; major alterations to the Indian monsoon and desertification of increased areas of the African continent.
As carbon dioxide levels increase, all models show an increase in the temperature of the earth and sea. In turn, this increases the amount of water vapour in the atmosphere, which is a greenhouse gas itself and will further raise the temperature. This is a big positive feedback. However, this water vapour can form clouds, and the clouds can have opposite effects: they can act as a blanket, adding to the greenhouse effect, or they can reflect sunlight back into space, especially high white clouds, and cause cooling. This is difficult to model.
loss of forest
Forests remove carbon dioxide from the atmosphere. Deforestation as a result of human activity is a well-known problem. But sophisticated models now show that decreased rainfall is likely to accompany global warming in some critical forested areas. These include the Amazonian tropical rainforests. Reduced rainfall could lead to dryer conditions and increased numbers of forest fires. This would increase global warming quite substantially.
acidification and warming of the oceans
As the carbon dioxide level in the atmosphere increases, so it also increases in the oceans, which raises the ocean acidity (lowers the pH). Effects on coral reefs and plankton populations have already been noted. Publications that highlight the causes and consequences of the rapid loss of the reefs now occurring around the world include those of Bryant et al. (1998), who estimate that coral reefs provide support to ecosystems worth more than $375 billion per annum to the global economy, and, very recently, Pandolfi et al. (2005), who provide an update on the degradation of ecosystems of 17 coral reefs around the world. The wider impact on marine life and on the food chain, including the human food chain, is an area requiring urgent further study.
global dimming as a result of aerosols and pollutants
Careful measurements of solar intensity indicate a reduction in sunlight over the past four decades. This is attributable to pollution generated by advanced societies, sulphurous coal in power stations and cars being the major sources. This cooling effect could have masked the true extent of global warming. The clean-up process in power stations, car exhausts and other sectors of industry is reducing the aerosol content of the atmosphere, and this could lead to greater levels of global warming than we have anticipated.
We know that extreme events will always occur, with or without global warming. However, the latest evidence suggests that the European heatwave of 2003 can be substantially attributed to human-induced global warming. The number of estimated deaths from that heatwave stands at some 25 000 and direct costs stand at some $13·5 billion. A statistical analysis by Schär et al. (2004) demonstrates that the mean summer temperature of 2003 was 5·2 standard deviations from the mean of the Gaussian through all the other summers over the period 1864–2003. On this basis it was a 1 in 1000 years event. However, taking subsets allowing for global warming over that period of time shows that this becomes a more likely event. Figure 1, taken from a second statistical analysis (Stott, Stone & Allen 2004) shows, in black, the average central European summer temperatures dating back to 1900. Clearly the summer of 2003 was an extreme event. However, in green the authors show the results of their model calculations of central European summer temperatures over the same period taking into account natural factors only, i.e. removing the influence of increased greenhouse gases on the temperature. On this green curve the summer of 2003 is about 2·3° above the norm. The authors conclude with 90% certainty that roughly one-half of the severity of that summer can be attributed to global warming.
There are likely to be significant biological impacts associated with climate change. There is clear evidence of changes in plankton populations in the north-east Atlantic. From the data of Beaugrand et al. (2002, 2003), the distribution, numbers and concentrations between 1958 and 2002 of warm temperate species and sub-Arctic species show distribution changes that are consistent with sea temperature rises. The temperature changes are affecting these populations and in addition there will be knock-on effects for a whole range of other populations. So, for example, if we look at the volume of cod and compare it with that of plankton (taking the specific plankton that the cod feed on), we see the well-known oscillations between the predator (cod) and prey (plankton) populations, a phenomenon that is very well understood. Taking into account the predator–prey relationship between cod and plankton we might expect the plankton levels to rise as cod levels fall. However, there is no evidence of that occurring; on the contrary, from 1985 a continuous fall in the plankton level is followed by a fall in cod population. This suggests that global warming may be the key to loss in the cod population. There is another interesting point to consider here: over this period carbon dioxide levels have risen substantially, and therefore absorption of carbon dioxide in the water has increased and acidity levels have risen. Is this having an effect on the plankton levels, and what about the food chain as a whole? These are important questions for ecologists to address, especially taking into account concern about over-fishing in this area.
Considering biodiversity further, the World Wildlife Fund Living Planet Index was published in 2002 (World Wildlife Fund 2002). This indicates a 30% loss of biodiversity over the period 1970–2000. In this study climate change is ranked towards the bottom of the causes identified for declining biodiversity. Of course the questions are whether this is the correct assessment for the impact of relatively rapid climate change on biodiversity and how these impacts are likely to change over the next few decades.
Adaptation and mitigation
There is a wealth of evidence to show that global warming is happening. Under the circumstances doing nothing is not an option: action is required on two fronts, mitigation and adaptation. We need to actively reduce our dependence on fossil fuels, moving to a low-cost, carbon-free energy system, focusing on renewables and on energy-efficiency gains. However, because of the considerable inertia in the Earth's system we will have to adapt to some global warming impacts over the next 30–40 years, whatever action we take.
An example of the sort of groundbreaking work we are doing on adaptation in the UK is the Office of Science and Technology's Foresight Flood and Coastal Defence project (Office of Science and Technology 2004). This is a project I commissioned to produce a long-term (out to 100 years) analysis of future flood risk in the UK. The project involved about 90 experts (engineers, risk analysts, socio-economists, modellers, environmentalists and many others) to assess the size of the future flooding problem for the UK, and to explore a range of responses within a framework of future scenarios. The work was sponsored at ministerial level by the Department for Environment Food and Rural Affairs (Defra) and supported by other government departments and non-governmental organizations.
Because the future is uncertain, the study assessed four possible future scenarios modelling different amounts of climate change and different socio-economic patterns for the UK. This is the most detailed analysis of its kind undertaken of the increased flood risks arising from global warming. Figure 2 is a snapshot of one part of the analyses and shows the change in risk levels under a worst-case scenario and a best-case scenario. In both scenarios the level of risk increases against a baseline assumption that nothing is done to improve flood management. Of course action is being taken and the government is investing an increasing amount of money in this area. However, the value of this analysis is that it provides a long-term indication of where action is required. We are developing a basket of responses, which are being tested within the scenarios. We will work with Defra on delivery of the project's future action plan.
The Kyoto Protocol is welcome and the UK is fully behind the process; however, it is not enough in itself. It is the beginning of a process that we will have to continue to develop. It is the first step to attaching a fiscal value to not emitting carbon dioxide. This will be a key driver in reducing emissions and, once it is running, the process will have to be ratcheted-up so that we can bring emissions sufficiently under control. A major challenge is moving into this second period, which will require commitment from the developed world and also from the fast-growing economies.
Transport is responsible for about 25% of our carbon dioxide emissions and we need to consider the transport system as a whole and take this problem on board. The combustion engine has become more and more efficient over time both in terms of kilometres per litre of fuel and emissions. Motor manufacturers are already developing hybrid vehicles and combined power trains and no doubt we will see full ‘drive-by-wire’ zero-emission cars in the future. A particular area of the transport system to consider is aviation. Air transport has become relatively cheap in recent years, and in part this is because there is no mechanism for applying tax to aviation fuel. This is an issue that needs to be considered in terms of the international regulatory system.
The built environment is extremely important: it consumes 50% of our energy, and nearly 50% of carbon dioxide emissions in the UK are caused by the building, maintenance and occupation of buildings. Improving building design to maximize energy efficiency and minimize energy use is a win/win situation: over the lifetime of a building its running costs are reduced and the emissions minimized. There is significant headroom for the UK to explore combined heat and power (CHP) systems; the benchmark here is Denmark, which produces about 50% of its electricity from CHP. In the UK the equivalent figure is 5·5% (Szokolay 2004).
In the UK we are working on every front to improve energy efficiency and increase the use of renewables and low carbon dioxide-emitting energy sources. Working also to ensure security of supply, these two things go hand in hand. Diversifying the energy supply also makes it more secure. The UK government is committed to reducing carbon dioxide emissions by 60% by around 2050, and the prime minister has announced that during the UK's presidency of the G8 group of nations we will focus on two issues: climate change and Africa. This is a global problem that will require a global solution, but the UK is providing active leadership in this area. Action is affordable: inaction is certainly not.