There is an excellent new science blog http://scienceofdoom.com/ about climate science which honestly sticks to the physics and avoids politics. In my opinion it is an improvement on http://realclimate.com which although good scientifically, has a tendency to indulge in attacking skeptics while defending questionable science like the famous hockey stick. The series of articles on CO2 as a greenhouse gas I would recommend to anyone interested in the greenhouse effect, and they have certainly helped me to better understand things.
So where are we in simple terms:
- Doubling CO2 from pre-industrial levels is predicted to raise average global temperatures 1.2 degrees centigrade. The physics here is pretty clear. Humans have currently added about 40% more CO2 since 1700 and temperatures have apparently risen by about 0.6 degrees. Counter arguments are that this rise is both due to natural recovery from the little ice age and to urban heat effects in the measurements.
- The good news is that the (direct) greenhouse effect has a logarithimic dependence on CO2 concentration. That just means that each time the concentration of CO2 in the atmosphere doubles, the radiative forcing causes about another 1 degree average rise in global temperature. Currently we have about 390 ppm (parts per million) compared with about 280 ppm 200 years ago before the industrial revolution – so about a rise of one third. If the concentration were to continue rise to say 1200 ppm (4 times pre-industrial levels – which is very unlikely), then the direct greenhouse effect would cause a total of 2.4 degrees rise in temperature.
- The IPCC predictions are based on complex Global Circulation Models which include many other hypotheses about positive feedback of extra water vapour caused by evaporated from the oceans and a decreased albedo due to melting snow caps. These lead to far higher predictions for global warming, and even to “run away” global warming.
- Perhaps these feedback effects are exaggerated or simply wrong. Increased clouds lead to less incident solar energy to the surface and more cooling. Certainly water vapour is the dominant greenhouse gas at between 60 – 90% of the total effect so small changes in water vapour can have big effects. When water condenses as water droplets in clouds then on average clouds cool the planet. The physics here is as yet unclear, complex and probably impossible to predict anyway.
- One hint that things may not be so bad as some predict is that life on Earth has thrived over the last 4 billion years. There is ample geological and physical evidence that concentrations of CO2 have existed previously at levels many times higher than now. If dangerous positive feedbacks really exist leading to run away global warming, as for example happended on Venus, then surely this would have already have happened over the long history of the Earth.
- The unique feature of the Earth seems to me to be that there is a stable climatic balance between Water, life and geology – sometimes called Gaia theory. The oceans make up 60 % of the surface and are by far the main drivers of climate. Plate tectonics, driven by internal radioactive heat maintain the continents and readjust heat balance. Changes in the earth orbit parameters lead to ice ages when the continents are distributed over the poles as now. Life continues to maintain the balance of gases in the atmosphere. Oxygen and Carbon Dioxide are gases of life.
Many threats exist for life on Earth over the next few million years namely: massive meteor impacts, super volcanoes, ice ages etc. The main danger for us over the next thousand years is profligate overpopulation of humans leading to the destruction of the environment and species extinction. Global warming is a symptom of our evolutionary success not the cause of our predicted eventual downfall. Whatever happens in the next million years life on Earth will continue, recover and flourish with or without us, as it always has done.