(Nederlandse versie hier)
There are many “half truths”, which in principle are correct (and are also known to the climate scientists, and are taken into account in climate models), but which are often wrongly interpreted:
– Climate has changed in the past as well (and there were no humans to blame. So right now it’s probably a natural process as well.) The fact that climate has changed in the past due to natural forcings (no scientist would deny that), is no evidence whatsoever that the same is happening now. Natural factors (eg solar activity) are not suddenly turned off now, and they are included in climate models, but they can only explain a small fraction of the observed recent climate change. This is probably the most popular “skeptic” talking point, even though the logic of the argument is flawed. The fact that forest fires occur naturally, is not a valid alibi for Johnny who just caused a fire by throwing away a burning cigarette. This example also illustrates another logical fallacy: That we, insignificant human beings, could never influence something so large as the Earth’s climate. (As if we’re committing hubris by pointing out that CO2 traps infrared radiation.) While a burning cigarette can cause quite a forest fire. And in the history of the Earth, micro-organisms (still a lot smaller than humans) have had quite an influence on the changing climate.
– Nature emits more CO2 than humans (so therefore the human contribution to atmospheric CO2 -and thus to climate change- is negligible.) The first part is correct (and it happens to be part of my current research area), but the conclusions that are often drawn from it are way off the mark. Nature is in balance (emission and uptake of CO2 by the biosphere are roughly equal over multi-year timescales, except in times of climate change). An increase in CO2 emissions from another source, even though small compared to biogenic emissions, can indeed increase the atmospheric concentration. Indeed, it has been proven that the increase in CO2 is due to human emissions, using the fact that fossil (anthropogenic) CO2 has a different isotopic composition that (current) biogenic CO2. The funny thing in this “skeptical” line of thinking is that it implicitly assumes that CO2 affects climate.
– Water vapor is the most important greenhouse gas (dwarfing the effect of CO2.) Again, even though the first statement is in principle correct, the second part is dead wrong. Water vapor doesn’t change due to increased emissions, but it changes in response to the increase in temperature: warm air can hold more water vapor. Therefore water vapor works as a positive feedback: If the climate warms, it causes even more warming, and vice versa. Water vapor is in a fast equilibrium with the biosphere, and it is not a climate forcing (cause), but rather a feedback (reinforcing effect). If we wouldn’t have changed the climate, water vapor would not have changed either. The effect of water vapor is definitely included in climate models: A large part of the warming from CO2 occurs indirectly via water vapor as a positive feedback. See also here and here.
– At the end of the ice ages, temperature rose first, followed by CO2 ~800 years later (and thus CO2 is an effect of temperature rise rather than a cause.) However, both temperature and CO2 continued to rise for another ~4000 years. Thus, the initial temperature increase at the end of an ice age is not caused by CO2, but CO2 did contribute to the remaining temperature increase in the final ~4000 years. It acted as a positive feedback, similar as water vapor. Another major feedback mechanism at the end of the ice ages is due to the changing albedo (reflectivity) when ice sheets disappear and the darker land (or water) surface is exposed again. Temperature and CO2 influence each other in both directions; it is a bit of a chicken-egg discussion. However, the current situation is clearly different from that at the end of the ice ages, since now we know that the extra CO2 is brought into the atmosphere by human activity. Currently, CO2 is not increasing in response to the warming, but rather due to human emissions, and as such it is now one of the driving forces of the warming. You may have noticed that Al Gore doesn’t discuss the full complexity of the ice ages in “An inconvenient truth”. See also here.
– The greenhouse effect is natural (and thus humans have nothing to do with it.) First part is absolutely true, otherwise the average temperature on Earth would be well below zero degrees C, and life as we know it would not have been possible. But we are strengthening the natural greenhouse effect by increasing the amount of greenhouse gases in the atmosphere. This statement is actually a very strong argument that greenhouse gases indeed influence the temperature. And since it’s certain that we are increasing the greenhouse gases in the atmosphere, it is therefore clear that current climate change is indeed due to human emissions.