The Intergovernmental Panel on Climate Change is in the process of putting out its fifth series of assessment reports. There are three components in this series. The report on the first component – science – was released at the end of September. The other two – impacts and mitigation – will be published in March and April 2014. While we wait for the perspectives that those reports will provide on strategic responses, the assessments made in this one must provoke some thought on the trajectory of climate change over this century.
The report assesses available scientific evidence for seven major climate phenomena. Looking at historical patterns, it assigns a likelihood to whether a change in trend has taken place since 1950 and whether this is the consequence of human actions. Looking ahead, it makes a judgement on the prospects of these trends being reinforced over two time horizons – early 21st century and later 21st century.
Looking back over the past six decades, two out of the seven phenomena are seen to have had a high likelihood of intensification. These are “warmer and/or fewer cold days” and “warmer and/or more hot days” over land areas. Further, the human contribution to both these changes in trend is seen to have been very likely. The scientific evidence apparently is far less persuasive on the other five phenomena. Some are likely to have seen a change in trend over this period, while there is low confidence in reaching this inference in others. In one, “increase in intense tropical cyclonic activity”, while the global trend is indiscernible, it is virtually certain to have happened in the North Atlantic – last year’s Hurricane Sandy offers a vivid illustration of this!
Does the historical pattern matter? In particular, are predictions of future trends critically dependent on the precision with which models have explained past dynamics? This is always a dilemma in forecasting, whatever the domain of variables. Be that as it may, the mandate of the report is to assess the likelihood of these trends being reinforced over the course of the current century and this has been done. Keeping in mind all the caveats about forecast accuracy, there is a striking pattern to the forecasts. For the early 21st centu