The synthesis report from the Copenhagen climate congress of scientists held in March has been released. It updates the 2007 IPCC report in the light of the latest developments in the science, and means that the UN Copenhagen conference in December will have an overview of the state of our current understanding of climate change. Â
The report has six sections. The first deals with climatic trends and is blunt. Greenhouse gas emissions and many aspects of the climate are already changing near the upper boundary of the IPCC range of projections.Â In the case of sea level rise the rate is even greater than indicated by the IPCC projections.Â
By far the largest amount of heat stored at the Earthâ€™s surface is found in the ocean, not the atmosphere. Although the oceanÂ warms more slowly thanÂ the atmosphere, a change in ocean temperature is a better indicator of change in the climate than changes in air temperature. Current estimates indicate that ocean warming is about 50% greater than had previously been reported by the IPCC. This helps better explain the trend in sea level rise, the rate of which has increased in the period from 1993 to the present.Â Looking ahead to 2100,Â projections that do not include possible changes in polar ice sheet movement now suggest a sea level rise of around a metre or more.Â (The top of the worst scenario IPCC range was 59 cms, though that did not include feedback uncertainties or possible changes in ice sheet flow)
Importantly the report points out that sea level rise will not stop in 2100. Changes in ocean heat content will continue to affect sea-level rise for several centuries at least. Melting and dynamic ice loss in Antarctica and Greenland will also continue for centuries into the future.Â In fact, global average surface temperature will hardly drop in the first thousand years after greenhouse gas emissions are cut to zero.
The unexpectedly dramatic reduction in the area of Arctic sea ice cover is important for climate on a large scale because of the feedback effect of the lowering of reflection of the sunâ€™s radiation. Increased water vapour following warmer global temperature is a further amplifying feedback.
Atmospheric CO2 as well as methane and nitrous oxide concentrations have increased dramatically over recent decades because of human activities. Ice core and sediment records show that the concentration of all of these gases in the atmosphere is now higher than it has been since long before modern humans evolved. In fact, the CO2 concentration in the atmosphere has not been substantially higher than it is now for at least the last 20 million years of the Earthâ€™s history.Â
At this point the report draws attention to a further amplifying feedback relating to the land and ocean sinks which have so far removed over half the CO2 released into the atmosphere through human activities.Â A weakening of the sinks is already occurring and there is likelihood that the decrease in the fraction of CO2 removed from the atmosphere by sinks will become larger under high future emissions scenarios. Several effects contribute to this likelihood, including increasing ocean acidification, ocean circulation changes, and water, temperature, and nutrient constraints on land CO2 uptake. Also, previously inert carbon pools can be mobilised and released into the atmosphere either as CO2 or methane, a more potent greenhouse gas. Two areas of concern are tropical peatland carbon, which is vulnerable to land clearing and drainage, and the large stores of organic carbon in Arctic permafrost, which are vulnerable to warming.
Finally, on smaller scales, the report refers to the observed increase in extreme events.
The congress was not an official UN gathering.Â It was organised by the International Alliance of Research Universities with the aim of bringing together the new knowledge that has emerged since the IPCC report.Â Open to all, the congress was attended by approximately 2500 people, most of them researchers.Â They came from nearly 80 different countries and contributed more than 1400 scientific presentations.Â The synthesis report, produced by a writing team of twelve, has been extensively critically reviewed â€œto ensure that the messages contained in the report are solidly and accurately based on the new research produced since the last IPCC Report, and that they faithfully reflect the most recent work of the international climate change research community.â€
The language of the report is fully accessible to a wide audience.Â No one can go to Copenhagen in December claiming that the scientific picture is confusing.Â Â
Hot Topic will highlight other sections of the report in further posts.