After three years of study, an intergovernmental panel of the United Nations has summarized its Fourth assessment of climate change since 1990.  The latest report exudes more confidence that recent climate changes are likely caused by human activities, especially by the release of greenhouse gases to the atmosphere. 

 

Direct observation of climate warming

     “Warming of the climate system is unequivocal,” the report intones. Eleven of the 12 most recent years rank in the warmest 12 years in the century-and-a-half of instrumented observations.  The upward trend of surface temperature is accelerating: the trend over 100 years ending in the year 2000 was +0.6°C, but the trend from 1905 to 2005 was +0.74°C, because the last six years have been abnormally warm.

 

     Previously reported differences between warming at the surface of Earth and in the free atmosphere “have been largely reconciled.”  Errors in temperature measurements by both satellite and balloon borne sensors have been identified and corrected.  The warming in the free atmosphere is now seen as comparable to the warming at the ground.

 

     The ocean has been absorbing over 80% of the heat added by the warming atmosphere, and ocean temperatures down to a depth of 3000 meters have been rising since 1961. This was one cause of sea level rise, which was observed to be +17 cm in the 20^th Century. The rate of sea level rise nearly doubled in the period from 1993 to 2003.

 

     Greater losses of ice from the ice shelves of Greenland and Antarctica may have something to do with that recent surge.  Not only has melting increased, but the flow speed of some outlet glaciers which drain ice from the interior of the two land masses has increased, and the ice sheet has also thinned in places.

 

Changes on a regional scale

     Arctic temperatures have increased at twice the average global rate in the last 100 years; late-summer extent of sea ice in the Arctic has decreased by 7.4% per decade, since 1978.

     Precipitation has increased in eastern North America, northern Europe, and central Asia.  Less precipitation has been observed in the Mediterranean basin, southern Africa and the Sahel.

     Storms with heavy precipitation have been observed more often in most land areas.

     While there is more evidence of intense tropical cyclones (especially hurricanes) in the North Atlantic Ocean since 1970,  “there is no clear trend in the annual numbers of tropical cyclones” around the world.

 

Modern climate in the context of ancient climates

     The past 50 years are likely the warmest 50-year period in the past 1300 years, insofar as science can infer what the temperatures were before instruments were used.  The last time that the polar regions were warmer than they are today, the sea level was 4 to 6 meters higher than it is now.  That period was 125,000 years ago.

 

What drives climate change?

     The IPCC assigns the main drivers of climate change to be ever-growing levels of carbon dioxide (CO₂), methane, and some other gases, plus changes in land-use. The concentration of CO₂ in the atmosphere in 2005 was higher than it has been during the last 650,000 years, as inferred from ice cores. The increasing presence of CO₂ results first from burning of fossil fuels, and secondly from changes in land use.  Concentrations of methane have more than doubled in the industrial age, as a result of agricultural activity and use of fossil fuels. The third most important greenhouse gas, nitrous oxide, has also been increasing because of agriculture.

     The effect of the greenhouse gases mentioned above has been to create an imbalance between energy coming into the Earth and atmosphere, and energy leaving the Earth and atmosphere.  The imbalance now amounts to 2.3 Watts of excess incoming energy, per square meter of ground surface; the imbalance is called a radiative forcing.

 

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     The contribution of carbon dioxide to this imbalance has increased by +20% in the last ten years, the largest change for any decade in at least 200 years.

 

Attributing causes for climate change

     IPCC says that the widespread warming of the last 50 years, together with the loss of ice, is extremely unlikely to have resulted from purely natural variations within the atmosphere and ocean; and very likely not due to natural causes alone. They conclude then the cause is very likely the increasing greenhouse gas amounts which are introduced into the air by human activity. Human influences now can be detected in the patterns of ocean warming, of winds, and of extreme temperatures.

 

     The lower atmosphere (the troposphere) which extends up to about 10 km, has warmed while the stratosphere above it has cooled.  This pattern is “very likely due to greenhouse gas increases plus the depletion of ozone.”

 

     Temperatures on the hottest nights, the coldest nights, and the coldest days have increased, and that is likely due to greenhouse effects.  Also, such effects are “more likely than not” to have increased the risk of heat waves.

 

     The overall sensitivity of climate to changing levels of greenhouse gases has not been accurately known before this report.  This new IPCC report appraises the global average warming to be from 2° to 4.5°C, and the most likely value to be 3°C, when the concentrations of CO₂ are doubled from natural levels that prevailed before the industrial age.   Six years ago the IPCC gave a wider range of values, and did not give a best estimate.

     Finally, the climatic variations of temperature in the seven centuries from 1250 to 1950 are now said to be caused by volcanoes and changes in the output of the sun, plus some human influence in the first half of the 20th Century.

 

Some climates have NOT changed.

     There is no significant trend in sea ice around Antarctica (in contrast to the Arctic).  Nor is there enough evidence for any trends in small phenomena like hail or tornadoes, or for any changes in the grand circulation of the Oceans (the “meridional overturning”).

 

Projections of Future Changes

     Though the IPCC considered a wide range of projections of future economic trends and emissions of greenhouse gases, little difference is expected in global temperatures in most of these projections until the year 2030.  For the next 25 years, the global temperature trend is mainly affected by warmth already stored in the Ocean and by greenhouse gases already in the atmosphere.  Until 2030, the Report projects a warming trend of 0.2°C per decade; this is close to the rate of warming observed since 1990.

 

     Even if greenhouse gases could somehow be maintained at the concentration of the year 2000 (which would require drastic cuts in emission of these gases), the warming trend would still be +0.1°C per decade. That means that half of the warming over the next 25 years results from the climate lag caused by greenhouse gases already in the air by the year 2000.

 

     Emissions at current rates would cause further warming (after 2030) and many kinds of changes on Earth that will very likely be larger than the changes seen in the 20th Century.   The new Report claims to provide more advanced projections than the previous (2001) report, as "it relies on a larger number of climate models of increasing complexity and realism, and . . .  new feedbacks from the carbon cycle."

 

     The Report emphasizes an often overlooked fact, that further warming diminishes the ability of both the land and the Ocean to absorb carbon dioxide.

 

 

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High Points of new IPCC Fourth Assessment Report

 

Warming “very likely related” to activities of mankind