The Physical Science Basis of Climate Change:
2007 IPCC Working Group I Report
Working Group I of the Fourth IPCC (Intergovernmental Panel on Climate Change Assessment Report states that “warming of the climate system is unequivocal.” The average increase in global surface temperature over the past 100 years is 0.74 degree Centigrade (C). Warming rates of the lower- and mid-tropospheric layers of the atmosphere are now confirmed to be similar to those of the surface temperature.
The IPCC’s Fourth Assessment Report, released in November 2007, consists of three main “working groups,” each group focusing on a different aspect of climate change and its effects. Working Group I concentrated on the “Physical Science Basis” of climate change, including drivers of change, climate processes, observed changes, and estimates of future change. The information in this most recent report builds on previous IPCC assessments, as modified by new research from the past several years.
Also, human activity has likely influenced the changes in extra-tropical storm tracks and temperature patterns in both hemispheres, according to the report.
Counteracting this warming effect, to some extent, is the cooling effect of aerosols (sulphate, organic carbon, black carbon, nitrate, and dust). However, atmospheric levels of aerosols have been decreasing. In balance, the warming effect of long-lived greenhouse gases has outweighed the cooling effect of aerosols, according to the report.
Increases in Greenhouse Gas Concentrations
Working Group I’s report states it is very unlikely that global climate change can be explained through natural causes alone. It is very likely that most of the observed increase in global temperature since the mid-20th century is due to the increase in greenhouse gas concentrations from human activity.
Carbon dioxide (CO2), methane, and nitrous oxide are all important greenhouse gases. Since 1750, atmospheric concentrations of greenhouses gases (GHGs) have clearly increased as a result of human activity, states the Working Group I report. Burning of fossil fuels has been the greatest source of increase in CO2, according to the report. Land-use change has had a smaller but significant contribution to CO2 concentrations. Ice cores have determined that methane levels in the atmosphere have exceeded the natural range over the last 650,000 years. Methane increases are due to human activities, including agriculture and fossil fuel use. In addition, more than a third of all nitrous oxide emissions stem from human activities, primarily agriculture.
According to the National Oceanic and Atmospheric Administration, water vapor is the most abundant GHG. Working Group I states that water vapor has increased consistently with warmer temperatures because warmer air holds more vapor.
Oceans have been absorbing more than 80 percent of the increasing atmospheric heat, causing sea levels to rise, states the report. Sea level rise is also attributed to the melting of mountain glaciers and ice caps. Working Group I’s scientists report that average arctic temperatures have increased at almost twice the global average rate in the past 100 years; on the other end of the globe, the Antarctic shows some localized changes, but it is predicted to remain cold enough to brace against significant melting.
Other observed trends include substantial increases in precipitation over some land areas; intensified droughts in the tropics and subtropics; more frequent heavy storms; a fewer number of cold days, nights, and frosts; and a higher frequency of hot days, nights, and heat waves.
For future predictions, all three working groups modeled
four scenarios that illustrate possible future societal values and rates of
population growth. Taking a range of possible higher emission scenarios into
account, the report projects a warming of about 0.2 degrees C per decade over
the next two decades. Furthermore, even if emission rates were held constant at
year 2000 levels, warming would still occur at the rate of 0.1 degree C per decade
over the next two decades because of the slow response of the oceans. Temperatures
are expected to increase more rapidly over land areas and in high northern
latitudes, and less quickly over the Southern Ocean and parts of the
Other predictions included in Working Group I’s report are more thawing in permafrost areas; decreasing snow cover; more frequent heat waves and heavy precipitation events; more intense hurricanes; and changes in wind, precipitation, and temperature patterns as a result of tropical storms moving poleward.
Finally, the report projects the rise in sea level to continue for centuries due to the time scales associated with climate processes and feedbacks, even in greenhouse has concentrations were to be stabilized.
-- Katie Starzec, CASMGS Communications
-- Steve Watson, CASMGS Communications