A student has sent me five questions on the issue of man-made climate change. As they may be seen as typical, I answer them here on the Klimazwiebel. This may be useful – for the student, who will be able to read the comments of the readers of the blog, – for the bloggers to better understand the issues and concerns among lay people, – for me, who will have a critical assessment of my response to the student.
The idea with this post is that the student will read it plus the comments - so that the different contributions should not represent too much piecework. Therefore, commenters, deal just with the questions raised, and try to make only one statement so that the material, which eventually gathers, is compact!
1. Human activities contribute to global warming – Yes, the major human influence on global warming is through the emission of so-called greenhouse gases (GHGs), in particular carbon dioxide, which accumulate in the atmosphere und remain there for several decades. The air temperature (near Earth surface) has risen since pre-industrial times (ca. 1850) by about 0.7 oC; this warming can not be explained by internal climate variability alone, but can only be understood as a result of contributing external factors, among them elevated GHG levels in the atmosphere. It is thought that most of the warming, in particular the strong warming since about 1970, can be explained only by the increased concentration of GHGs.
2. There are no practical solutions available to stop the current warming (man-made or not). – In the climate system there is some "inertia": developments "persist" for a while, so that any efforts to reduce the GHG-related warming (either the ongoing multi-decadal increase of temperature, or the level of temperature) will need at least two, three or even more decades of years to take effect. Thus a short term "stopping of the warming" or an "ending of the elevated temperatures and a return to preindustrial conditions" is not conceivable. – One issue, related to your question, is that of "geoengineering", in particular man-made modifications of the geophysical properties of the Earth surface or of the atmosphere, which would change the "radiative balance" (e.g., reflecting substances in the lower stratosphere). Research to this end is underway, but at this time, the public opposition to such efforts is massive, it seems. – If the current climate protection policy would have been successful in initiating a policy, which would really limit the change of global mean temperature to 2oC (relative to pre-industrial conditions; of which 0.7oC have already taken place), then a significant stress on societies and eco-systems would emerge, which would need broad efforts to manage the adaptation of societies and ecosystems to these changed conditions.
3. When a critical level of the temperature is passed, the warming of Earth can not longer be stopped. – You are referring to the "runaway effect", which is thought to have taken place of Venus, where temperature are very high now. I know little about this issue, but I would believe that in principle there may be such a critical level, but our climate models describe all futures with elevated GHG levels without such a runaway effect.
4. Earth has no mechanisms to counteract the current warming. – Again, this depends on the time horizon you are looking at. When dealing with horizons of, say 100 years, there is none; the effect of GHGs on the radiative balance is simply too strong. However, most (not all) of the additional GHGs in the atmosphere will disappear in the long run (of many hundreds of years) by sedimentation in the deep ocean and other mechanisms, so that on the long term Earth will return to its variations enforced by the configuration of its orbit relative to the sun.
5. The warming dramatically affects ecosystems, humans and their activities. – This depends on the intensity and the rate of warming. The warming, we have seen so far, had little effect (even if interested parties try to relate all kind of extremes, e.g., Katrina to the warming). The slower the warming, the easier the autonomous adaptation will be. – Locally there can be dramatic effects: if, for instance, a glacier is melting in the Alps, the ecosystem in that region will change; the water supply will change; tourism will be affected. Rising sea level will in general have various adverse effects; increasing the risk of inundation; forcing more efforts in coastal defence; problem with groundwater and salinity, coastal erosion. On the other hand, humans have always been ingenious when confronted with new challenges, and if enough time is available, one may expect this also in this century. (We may confidently predict the emergence of unexpected, entirely new knowledge and options in the course of this century.) – Also keep in mind that climate is just one changing condition of our life; many other things change at the same time: technology, social organisation, economic wealth and options, social values. Which of these changes will be the most important in 2050, for instance?