Global warming has attained significant levels
If we accept evidence that the temperature is rising, and that this is caused by the concentration of greenhouse gases, the next step is to ask by how much and whether, if the observed trend persists, the current level and the expected increase will cause noticeable changes in the physical world.
Records show that the average global temperature rose by 0.85 ºC between 1880 and 2012, and the models forecast that it might increase by up to 4.8 ºC by the end of this century, assuming no changes are made. It is difficult to grasp what these changes mean since we are accustomed to much greater temperature changes between day and night, and between summer and winter. First, we must bear in mind that these are global average temperatures. These figures become more meaningful if we consider that 5 °C is the difference between the current global mean temperature and that during the last glacial period.
Moreover, natural systems are hugely complex and it is very difficult to model all the factors involved. The models are constantly being reviewed and fine-tuned, but they are necessarily incomplete. In fact, because of its approach of extreme prudence, the IPCC has been accused of downplaying the speed with which changes may occur.
Because the decline of the Earth's albedo due to melting of surface ice will cause more radiation to be absorbed (ice reflects solar radiation much more than the darker land or ocean), increasing surface temperatures and further accelerating ice melting. Also, the thawing of permafrost (the layer of the Earth's surface that is permanently frozen) in, for example, large areas of Siberia, will release methane retained in the peat; methane is a much more powerful greenhouse gas than CO₂, and this will contribute to additional permafrost melting, creating a runaway effect. Rainforests, which are currently carbon sinks, might begin to deteriorate due to an increase in temperature, which would lead to a massive release of CO₂, contributing to additional warming. Methane clathrates, crystalline structures that trap methane in sheets of ice located beneath the sediment on the bed of the oceans and cold, deep lakes, may become unstable as outside temperatures rise, which would trigger the release of methane, leading to acceleration of warming that might destabilise more methane clathrates, which would accelerate the phenomenon. Work is still being done to measure how these factors would accelerate the current (more conservative) projections.
Guest post written by Carmen Becerril Martínez, External Director from ACCIONA, and Magdalena García Mora, Manager of Analysis of Energy policies and Climate Change from ACCIONA.