NIWA Media Statement 3 February 2007

Global warming “unequivocal”: IPCC assessment

In its strongest statement yet, the Intergovernmental Panel on Climate Change (IPCC) says there is now unequivocal evidence of global warming.

A plenary session involving government representatives and scientists from over 130 countries has just accepted the first in a series of reports which make up the IPCC’s Fourth Assessment, and approved its Summary for Policymakers. This first report, from the IPCC Working Group 1, deals with the physical science basis of climate change.

Dr David Wratt of the National Institute of Water & Atmospheric Research (NIWA) is in Paris for the plenary.

“For every continent except Antarctica there is good evidence that the climate is warming and the cause cannot simply be natural variation,” says Dr Wratt.

It has been six years since the IPCC’s Third Assessment in 2001. “Every time the IPCC has assessed the scientific literature it has become clearer that human activities are changing the climate. After all the research effort of the past six years, the broad picture from the previous assessment has been strengthened,” says Dr Wratt.

The IPCC now says it is “very likely” [more than a 90% probability] that most of the global warming since the mid-20th century was due to increased greenhouse gases from human activity. The previous report put the probability at “likely” [more than 66%].

The report says it is “virtually certain” [more than a 99% probability] that most land areas will experience fewer cold days and nights, and more hot days and nights, over the course of the 21st century. Reinforcing expectations that climate change will bring more weather extremes, the report says it is “very likely” [more than 90% probability] that the frequency of heat waves and very heavy rainfall events will both increase over most land areas. The areas affected by droughts, the incidence of extreme high sea level, and intense tropical cyclone activity are classified as “likely” [more than 66% probability] to increase.

The report examines a range of scenarios for greenhouse gas emissions. Those that do not assume additional climate initiatives, such as implementation of the UN Framework Convention on Climate Change, would see us reach atmospheric concentrations of 600–1550 parts per million (ppm) by 2100. The “best estimates” for projected global temperature rises in 2090 compared to 1990 range from 1.8 °C for the lowest of these scenarios to 4.0 °C for the highest scenario. For the highest of these scenarios, the “likely range” of warming (which takes model uncertainties into account), is 2.4 to 6.4 °C.

“Assessment of the likely impacts of various greenhouse gas scenarios and associated temperature increases is within the scope of the IPCC’s Working Group 2, whose report is due out in April,” says Dr Wratt.

The present report projects globally-averaged sea level rises of between 0.19 and 0.58 metres by the end of this century, relative to 1980–1999 levels, for emission scenarios without additional climate initiatives... These estimates could be higher if rates of discharge from the Greenland and Antarctic ice sheets continue to increase. .

In other projections:
• Ocean acidification is expected to accelerate, with a 0.14–0.35 units reduction in pH at the ocean surface over the rest of the 21st century. Since pre-industrial times, the ocean pH has decreased by 0.1 units. (The lower the pH, the more acidic).
• Warming is expected to be greatest over land and at most high northern latitudes (i.e., near the Arctic), and least over the Southern Ocean and parts of the North Atlantic Ocean.
• Snow cover is projected to contract.
• Sea ice is projected to shrink in both the Arctic and Antarctic. In some projections, Arctic late-summer sea ice disappears entirely by the latter part of the 21st century.
• The meridional (north-south) overturning circulation of the Atlantic Ocean (part of what is sometimes called the “thermohaline circulation” or the “ocean conveyer belt”) is “very likely” to slow but “very unlikely” to shut down entirely by 2100.

The report also considers possible changes further into the future. Even if greenhouse gas concentrations are eventually stabilised at constant values, temperature and sea level rise are expected to continue increasing for centuries. Climate models suggest that a global average warming in excess of 1.9 to 4.6°C (depending on the model) above pre-industrial values, if sustained for millennia, would eventually lead to virtually complete elimination of the Greenland ice sheet and a sea level rise of about 7 metres. Modelled temperature changes in Greenland for this level of global warming are comparable with those inferred for the last interglacial period 125 000 years ago. Paleo-climate information suggests sea levels at that time were 4–6 metres higher.

“Despite the definitive nature of much of the evidence and analysis in this report, there is still more scientific work to be done,” says Dr Wratt. “For example, the report says that current climate models project the Antarctic ice sheet will remain too cold for widespread surface melting during the next few centuries, and will grow due to increased snowfall. However if ice flow processes continue to speed up, this might lead to an overall loss of Antarctic ice and a faster rise in sea level”.

Ends

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