The TAR report estimated the degree of global warming by the end of this century lies between 1.4 and 5.8° C. In part, the spread is due to uncertainty about the nature and strength of processes that could dampen or amplify the initial greenhouse gas forcing.

In addition to the focus on feedbacks associated with water vapour and clouds, however, research over the past few years has yielded a better understanding of three additional effects that were recognized as being important in the IPCC TAR but for which little quantitative information was available at the time.

• The radiative properties of aerosols. Estimates are now moving towards a higher cooling effect for aerosols—small particles suspended in the atmosphere that generally scatter incoming solar radiation and thus cool the Earth’s surface—than previously thought. In other words, as greenhouse gas concentrations increase and aerosol loadings are reduced, warming increases.

• A decrease in albedo. Albedo—the reflectivity of the Earth’s surface—is decreasing due to the melting of snow and ice, with the most extreme example being the Arctic Ocean. Retreating ice and snow expose darker underlying land and ocean surfaces, leading to enhanced absorption of sunlight and further warming.

• Changes in the terrestrial carbon cycle. 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 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.

—“Stronger Evidence but New Challenges”

The author of the report is Will Steffen, Executive Director of the International Geosphere-Biosphere Programme (IGBP) from 1998 through mid-2004 and, since then, IGBP Chief Scientist and Director of the Centre for Resource and Environmental Studies at the Australian National University.

I have drawn on the work of thousands of scientists around the world; however, the selection of the material to be incorporated in this synthesis, the emphases made and the inferences drawn are entirely my own. They do not necessarily represent the views of the IGBP, the Australian National University, the Australian Greenhouse Office or the Australian Government

—Will Steffen

Australia opted out of the Kyoto Protocol agreement. It is, however, on track to meet what would have been its obligation under the Kyoto terms—an 8% increase in greenhouse gas emissions above 1990 levels. The country is also part of the US-led Asia-Pacific Partnership on Clean Development and Climate.

Resources:

• Stronger Evidence but New Challenges: Climate Change Science 2001- 2005