The report of the Intergovernmental Panel on Climate Change (IPCC) represents a consensus of scientific assessments of a wide body of evidence. Some of the key elements that people need to understand are as follows:
- Carbon dioxide is a greenhouse gas. This means that it absorbs radiation from the sun and dissipates the energy by moving more energetically. This movement is what we feel as temperature.
- The concentration of carbon dioxide has increased from 285 ppm to over 400 ppm since the beginning of the 20th century. Climate models of increasing accuracy have estimated the effect of this on temperature and produced estimates of increases of several centigrade degrees. These models accurately describe past temperatures, and they agree that in the future the temperature will continue to rise, by anywhere from 2 to 5 centigrade degrees by the year 2100.
- The increased carbon dioxide in the atmosphere arises from combustion of fossil fuel. Scientists know this because they have done the math. The amount burned so far accounts for the increase in the concentration of the gas in the atmosphere. Another piece of evidence is that the extra carbon bears the signature of fossil fuels, in that it has less of the isotope carbon-14. This isotope is constantly produced in the upper atmosphere by irradiation of nitrogen gas. Plants pick this up and ultimately pass it along to animals and other organisms. When organisms die, the spontaneous breakdown of carbon-14 is not compensated by any further uptake of new carbon-14, so the amount of carbon-14 declines in their remains. Coal, gas, and petroleum are the remains of organisms long dead, and thus thoroughly depleted in carbon-14. So what do we expect to happen over the course of decades of burning this material? We expect the relative amount of carbon-14 in the atmosphere to decline. That is exactly what is happening, and furthermore the decrease agrees with the amounts of petroleum gas and coal that we have burned.
- Worldwide temperatures have increased by about one centigrade degree since the beginning of the 20th century, and in places the temperature rises have been considerably greater. The effects of this have been manifold: glaciers have melted, sea ice has decreased, tropical marine ecosystems have migrated away from the equator toward the poles, and the ocean itself has gotten warmer as it equilibrates with the air. There are many additional signs of the effects of increased global temperature cited in the IPCC report.
- The character of a biome, that is the organisms that live in a given area, is determined by temperature and rainfall. As temperatures rise worldwide, whole biomes will be affected. The response will be largely an increased number of extinctions, because, despite evolution’s tendency to favor the emergence of more adaptive forms, this emergence is generally not rapid, even for a single species, much less so for the large collections of species that make up a biome such as a tropical rain forest or a temperate deciduous forest like the one I live in. Evidence indicates that when there is an extinction like this that is spread over the whole planet, it takes about 10 million years for evolution to restore the original level of biodiversity. The new organisms will not just re-create the old ones. They will be very different for a host of reasons. The world of that distant era will be unrecognizable to us and possibly inconsistent with our survival, just as the world of today is different from that of the dinosaurs.