The latest issue of Scientific American has a informative article on the current state of climate science: why we know the earth is warming, and why we know human activity is partly to blame.
The authors are William Collins of UC-Berkeley; Robert Colman, an Australian; James Haywood of the UK's Met Office; Martin Manning of NOAA; and Philip Mote, the climatologist for the state of Washington.
Unfortunately it'll cost you $5 to read the article online. I'll summarize the key points here, but if you want to read the whole article you'll need to buy a copy or go to the library.
The main points:
Atmospheric concentrations of carbon dioxide, methane and nitrous oxide have been stable for 10,000 years — until they began growing rapidly about the time the Western world industrialized.
1. 11 of the past 12 years are the warmest since reliable records began around 1850. That's a pretty short time frame, geologically speaking, but the probability of that happening by chance are very small.
2. Measurements from ice cores and tree rings provide a longer time line, showing that the current climate is warmer than it has been for at least 1,300 years.
3. While natural variability occurs, temperature extremes have changed in accordance with the warming trends. Frost days and cold days have become less common, while heat waves and hot days have become more common.
4. The oceans are warming as well, more so at the surface than in the depths, a sign that the warming source is at the surface.
5. Overall, the planet's average temperature has risen .75 degrees Centigrade (about 1.5 degrees Fahrenheit) in the last 100 years — and the pace is accelerating.
Rising Sea Levels
1. The oceans have absorbed more than 80 percent of the added heat. This has warmed the water, which expands, causing sea levels to rise. Melting glaciers and ice sheets add to the effect.
2. The oceans have been measured rising an average of 3.1 millimeters a year. Over 50 years, that would mean a total increase of 155mm, or about 6 inches. The process is expected to accelerate, however, for a total rise in the 21st Century of maybe 40 centimeters (400mm, or about 16 inches) and possibly as much as 60 centimeters (about two feet).
3. With rising sea levels comes inundation of low-lying coastal areas, a higher water table, increased flooding, erosion, salinization of coastal waterways and wetlands, and greater danger from storms. An EPA study of the effects of various levels of sea rise suggests (while admitting it is an underestimate) that even a 6-inch increase would cost the United States alone something like $100 billion if we wanted to protect developed coastal areas and prevent inland flooding. That cost would be spread over 100 years, so the annual cost isn't too bad and assumes coastal development all but ceases, and that sea levels stop rising. The costs rise fairly rapidly with additional increases in sea levels.
We know humans are responsible for much of this increase for several reasons.
1. Some greenhouse gases, like halocarbons, have no natural sources.
2. Geographic differences in concentration comport well with human causation, with heavier concentrations over the more heavily populated and industrialized northern hemisphere.
3. Analysis of isotopes in atmospheric gas can identify the origin of the gas. It turns out most of it comes from burning fossil fuels.
4. There is more warming over land than over sea, and in the ocean the greatest warming is occurring at the surface — both indicators of a human factor.
5. The troposphere (the lower atmosphere) is warming while the stratosphere is cooling — exactly what you would expect if the cause was increased emissions of greenhouse gases and depletion of stratospheric ozone. If warming was primarily caused by solar activity, both layers of the atmosphere would warm up.
1. For the long-lived greenhouse gases, we know their heat-trapping effects fairly well, because we have precise measurements of their concentration and distribution in the atmosphere, and we know how they affect the planet's energy balance.
2. Five years have passed since the last major report, and in those five years temperature increases have been consistent with projections of greenhouse-driven warming.
3. The climate models used to make predictions and measure the effects of various warming and cooling factors are getting better. In addition, results are drawn from an ensemble of 18 modeling groups, so the weakness of any single model can be identified and its effect on conclusions reduced.
The article ends with a discussion of what isn't known, the limitations of current research and thus the lack of granularity in some areas. But overall I think it does a good job of explaining why leading scientists think humans are a significant factor in global warming.