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-20^th 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 21^st 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 21^st 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 21^st 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”.

Notes:

1. Download the Summary for Policymakers & view the IPCC official media conference: www.ipcc.ch
2. The Intergovernmental Panel on Climate Change (IPCC) was established in 1988. It produces regular, independent scientific assessments of the current state of knowledge on climate change. These include the major six-yearly assessment reports (2007 is the fourth), and special reports of a more or less technical nature. Membership of the IPCC is open to all member countries of the World Meteorological Organization and the UN Environment Program.
3. The 4^th Assessment involved:
   • 2500+ scientific expert reviewers
   • 800+ contributing authors
   • 450+ lead authors
   • 130+ countries
   Each chapter of the report has two Convening Lead Authors and a team of 7–15 Lead Authors, along with a host of contributing authors. Authors are selected based on their scientific expertise, with attention also paid to obtaining a geographical spread and a balance of specialist knowledge.
4. Each IPCC report goes through two formal rounds of external review. Authors must record how they deal with every review comment. Review editors, who are senior scientists in the field, ensure authors respond appropriately.
5. The 4^th Assessment Report is in four volumes:
   • Working Group 1, The physical science basis. [Just accepted by the Working Group Plenary]].
   • Working Group 2, Impacts, adaptation, & vulnerability, including a specific chapter on Australia and New Zealand. [Due to be accepted by WG2 on 6 April (Brussels)].
   • Working Group 3, Mitigation, including chapters on forestry and agriculture. [Due to be accepted 4 May (Bangkok)]
   • Synthesis Report. Integrates material from all 3 working groups in policy-relevant way. [Due to be accepted in November]
6. A Summary for Policymakers, which is finalised and approved at a Working Group Plenary, is a non-technical summary of a working group report The full scientific report, including a much longer Technical Summary, will not be available until mid-May, after the IPCC’s 26th Session formally accepts all three working group reports and the IPCC’s official publisher processes the material.
7. The IPCC does not carry out research nor does it monitor climate-related data. IPCC Assessments reference thousands of high quality & peer reviewed scientific studies conducted by research organisations around the world.

Warming of the climate system is “unequivocal”: other key facts

• Current global atmospheric concentrations of carbon dioxide and methane are the highest for at least 650,000 years.
• The increases in these greenhouse gases since 1750 (pre-industrial levels) are due primarily to emissions from fossil fuel use, agriculture, and land-use changes.
• There is “very high confidence” that human activity has warmed the globe since 1750. The best estimate for the radiative forcing from changes in solar output since 1750 is less than a tenth of the best estimate for forcing from human activities. (Radiative forcing is a measure of the importance of a potential climate change mechanism.)
• The increase in global average temperature between the latter part of the 19^th Century (1850-1899) and the last five years (2000-2005) was 0.76 °C (?0.19 °C).
• Eleven of the last 12 years rank among the 12 warmest years on record. The rate of warming for the last 50 years is almost twice that of the last 100 years.
• Global average sea level rose by an estimated 0.17 (?0.05) metres over the 20^th century, and this was faster than sea level rise in the 19^th century.
• Average Arctic temperatures increased by almost twice the global average rate in the past 100 years.
• Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7% per decade. Summer Arctic sea ice has shrunk by 7.4% per decade over the same period.
• Recent sea level rise (1993–2003) has been an average of about 3.1 mm/yr. The main contributors over this period are: expansion of seawater as the oceans warm (1.6 ?0.5 mm/yr), melting glaciers and ice caps on mountains in temperate regions (0.77 - 0.22 mm/yr), and shrinkage of Greenland and Antarctic ice sheets (0.41 - 0.35 mm/yr).
• Temperatures at the top of the Arctic permafrost layer have generally increased since the 1980s (by up to 3°C). The maximum area covered by seasonally frozen ground has decreased by about 7% in the Northern Hemisphere since 1900.
• Eastern parts of North and South America, northern Europe, and northern and central Asia have recorded significantly increased precipitation since 1900. The Sahel, the Mediterranean, southern Africa, and parts of southern Asia have got drier over the same period.
• More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics.
• Average North Hemisphere temperatures during the second half of the 20^th century were very likely [over 90% probability] warmer than during any other 50-year period in the last 500 years and likely [over 66% probability] the warmest in at least the past 1300 years.