Simple observation confirms the basic science of climate change. "All six years since the last report (2001 to 2006) are among the seven warmest years on record," notes Kevin Trenberth, head of the Climate Analysis Section at the U.S. National Center for Atmospheric Research (NCAR) and another lead author. "Northern Hemisphere snow cover has decreased and Arctic Sea ice has been at record low levels in the past three years."
In addition to such ice changes--accelerated melting in Greenland, western Antarctica and from mountain glaciers throughout the world--scientists have improved their understanding of the atmosphere's workings. For example, the tiny particles known as aerosols are far better understood, says atmospheric scientist Piers Forster of the University of Leeds in England andalso a lead author. "We estimate that their total radiative forcing is around -1.3 [watts/meter2]," which is a cooling effect, he says. "Because of this and a better understanding of how forcing terms add up, we are able to sum the radiative forcings and, for the first time, come up with the statement that we have very high confidence that humans have had a warming influence since preindustrial times." That influence continues via greenhouse gases from burning fossil fuels, and other sources; the temperature forcing from carbon dioxide levels has jumped 20 percent in just the last 10 years.
These observational improvements also extend into space, all the way to the sun, where scientists have used satellite data to better understand the amount of solar energy--and its impact here on Earth. "We therefore can make a comparison statement for the first time and say it is likely that solar forcing is at least five times smaller than the combined human influence," Forster continues. "Over the last 50 years, in particular, the natural forcing (solar plus volcanic) is most certainly negative. Meanwhile we've seen this large positive forcing from greenhouse gases."
"There are now three or four satellite temperature time series of the atmosphere, six years ago there was one. This duplication helped uncover some errors," adds lead author Thomas Peterson, a climate analyst with the U.S. National Ocean and Atmospheric Administration (NOAA). "Correction of that error made the time series show more warming and is part of the reason why you no longer hear skeptics say that satellites don't show any warming."
In addition, tide gauges, satellite measurements and some 1,250 data-collecting buoys have improved information on oceans, the most important heat sink on Earth. "The oceanographic community now has a system of vertical profiling floats distributed through most of the world oceans that tells us the vertical distributions of temperature and salinity every 10 days to depths of one to two kilometers," says Sydney Levitus, director of NOAA's World Data Center for Oceanography and another lead author. "Sea level observations are telling us that during the past 100 years sea level has risen at an average rate of 1.7 millimeters per year," most of that due to thermal expansion as the top 700 meters of the oceans warms and expands. In fact, a paper in today's Science reveals that the 2001 IPCC report underestimated sea level rise as well as the average surface temperature--land and sea combined--for the globe.
All of this data--and its conformance with predictions from computer-generated models--provide key evidence of climate change. "The fact that nature is confirming a posteriori the anticipation of models from 15 or 20 years ago is strong proof," Le Treut says. "It is very difficult to say that this is coincidental."
"The human signal has clearly emerged from the noise of natural variability," NCAR's Trenberth adds. "Numerous changes in climate have been observed at the scales of continents or ocean basins. These include wind patterns, precipitation, ocean salinity, sea ice, ice sheets and aspects of extreme weather."
This means that the science of climate change may partially undergo a shift of its own, moving from trying to prove it is a problem (it is now "very likely" that greenhouse gases in the atmosphere have already caused enough warming to trigger stronger droughts, heat waves, more and bigger forest fires and more extreme storms and flooding) to figuring out ways to fix it. "I would like to see a network of phenological data--such as bloom dates of particular plants--that could be tracked in real time worldwide," NOAA's Peterson says. "Temperature is a good parameter to measure. But the effects of the changes in temperature are relevant to measure directly, too."