Climate change science
Will Steffen, 2006
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Climate change science has made important gains since the IPCC Third Assessment Report (TAR). The evidence for a warming Earth is stronger and the impacts of climate change are becoming observable in some cases. The broader climate-related research community is beginning to tackle one of the most fundamental and difficult questions associated with the climate change issue – what constitutes “dangerous” climate change? Article 2 of the United Nations Framework Convention on Climate Change states that emissions of greenhouse gases should be limited to levels that avoid dangerous climate change. Although ultimately it is society’s role to determine what is dangerous and what is not, science can provide critical knowledge to underpin the societal debate. Recent advances in two major areas of research – determining by how much climate will change due to greenhouse gas forcing and observing and analysing the impacts of climate change – have thrown new light on the question of dangerous climate change.
Model-based estimates of the degree of global waming by the end of this century, lie between 1.4 and 5.8oC. In part, the spread in the range of estimates is due to the uncertainty about the nature and strength of processes that could dampen or amplify the initial greenhouse gas forcing. Most of the emphasis up to now has been on feedbacks associated with water vapour and clouds. Over the past few years, however, research has yielded a better understanding of three additional effects that were recognised as being important in the IPCC TAR but for which little quantitative information was available at the time.
The first of these effects is based on the radiative properties of aerosols, small particles suspended in the atmosphere that generally scatter incoming solar radiation and thus cool the Earth’s surface, acting in opposition to greenhouse gases. Estimates of the magnitude of the aerosol cooling effect have now been made, and the estimates are moving towards a higher value than earlier thought. This implies enhanced warming later this century as greenhouse gas concentrations increase and aerosol loadings are reduced. A second effect is associated with a decrease in albedo – the reflectivity of the Earth’s surface – caused by the melting of snow and ice. The most dramatic example of this effect will likely occur in the Arctic Ocean, which is now projected to become almost totally ice-free in summer late this century. Retreating ice and snow expose darker underlying land and ocean surfaces, leading to enhanced absorption of sunlight and further warming.
Thirdly, terrestrial carbon cycle dynamics are expected to change significantly through this century, with strong amplifying feedbacks to climate change. Several processes – the oxidation of soil organic matter, the number and areal extent of major disturbances such as fire, and the stability of carbon pools in wetlands and frozen soil – are all sensitive to climate. As temperature rises, these processes in general release further amounts of carbon to the atmosphere, forming a feedback loop that intensifies the warming.
Although much uncertainty still surrounds the timing, rate and magnitude of these effects, they all operate to amplify the initial greenhouse warming. Thus, there is now perceived to be a greater risk that the upper end of the well known IPCC TAR estimate of a 1.4 to 5.8°C temperature rise will be reached or exceeded by 2100.
The impacts of a changing climate are beginning to emerge. High temperature extremes, such as the August 2003 heatwave in central Europe that had severe impacts on human health, are becoming more common. Although any individual event cannot be attributed unequivocally to climate change, the probability of such high temperature events increases with the underlying trend of rising mean temperature. According to data from the reinsurance industry, the number of climate-related disasters has increased since about 1970. Impacts of a warming world on the Earth’s biological diversity are becoming evident, especially in alpine ecosystems and in the northern high latitudes. The number of bleaching events on the world’s coral reefs has increased strongly over the last two decades, and the combination of increasing acidity in the ocean and higher sea surface temperatures, both highly likely to increase further over the next half-century at least, will put more pressure on reefs.
Less clear are the links between climate change and two phenomena that could lead to potentially devastating impacts – droughts and tropical cyclones. Some regions of the Earth have experienced 50-year drying trends, with circumstantial evidence linking the drying with increasing sea surface temperature. Although there is no evidence for an increase in the number of tropical cyclones, some studies show an increase in the destructiveness of tropical cyclones, again related to the increase in sea surface temperature.
The observational evidence which supports the fundamental principles of climate change science has grown even stronger in the post-TAR years. The atmospheric concentration of CO2 continues to increase, and several lines of evidence, most notably isotopic analysis, attribute most of this increase to the combustion of fossil fuels. The instrumental record showing a warming Earth is supported by satellite measurements of tropospheric warming and by observations in the cryosphere and biosphere. The heat content of the upper layers of the ocean is increasing. A growing number of reconstructions of surface temperature over the past 1000 to 2000 years shows that the sharp temperature rise over the past century is now beyond the bounds of natural variability.
The imprint of greenhouse gases as the primary cause of the observed warming has also become clearer. The pattern of heat uptake in the world’s ocean basins agrees well with that simulated by climate models for greenhouse gas forcing. The observed moistening of the upper troposphere accords with expectations for greenhouse gas-driven changes in atmospheric water vapour content.
In summary, post IPCC TAR research has confirmed with stronger evidence the patterns of climate change described in the TAR. In addition, research over the past five years has taken a more systems-oriented approach to climate change, exploring the processes and feedbacks that affect the projections of climate change through the 21st century. A better understanding of the severity and rate of climate change over the coming decades is crucial to assessing the potential impacts on societies and ecosystems, some of which are now observable.
