The IPCC’s Fifth Report, September 2013. Some comments by Sir John Houghton


The Physical Science Component of the Intergovernmental Panel on Climate Change (IPCC)’s 5th report was agreed in Stockholm in the week beginning  23rd September 2013 and its Summary for Policymakers – about 30 pages long – was available on 27th September. The main messages are: 1) That it is extremely likely (i.e. more than 95% probability) that human influence on climate caused most of the observed increase in global average surface temperature from 1951- 2010. 2) That there is high confidence that this has led to warming of the ocean, melting of snow and ice, a rise in global mean sea level and to more climate extremes with increased intensity. 3) Further warming will result from continued emissions of greenhouse gases, causing  changes in all parts of the climate system. Considerable reductions in greenhouse gas emissions will be required if  climate change is to be limited. 4) Under almost all possible scenarios a rise of 1.5C is predicted by the end of this century relative to 1850 to 1900 temperatures, but in some scenarios the rise is greater than this. Most scenarios predict further warming beyond 2100.

Developed over 25 years

The work of the IPCC, since its formation 25 years ago, has seen large growth in the number of scientists involved, the amount and breadth of the science being addressed and the number of published papers relevant to the issue. Over this 25 year period, the coverage of observations of the climate over the globe has increased enormously. So has the development in understanding that has been possible through climate simulations using computer models with vastly increased power and sophistication. Putting the observations and computer simulations alongside each other has led to increased consistency in the descriptions of the human induced component of climate change in the past and increased confidence in climate projections for the future.

Is Global Warming Still Happening?

Recently in the media and otherwise, questions have been asked as to why the rise in global average temperature seems to have leveled off somewhat since the year 1998. Does this indicate that ‘global warming’ is no longer happening? To begin to answer that, it must be pointed out that there are many other indicators of a changing climate – for instance, rapidly decreasing sea-ice in the Arctic, increased occurrence and intensity of climate extremes such as heat waves, floods and droughts, and an average rate of sea level rise that since 1900 has increased from about 1 cm/decade to over 3 cm/decade. A further confirming indicator of a warming climate has come from recent observations of ocean temperatures. The oceans are a crucial integral part of the climate system; their thermal capacity is well over 100 times that of the atmosphere so although ocean temperatures change comparatively slowly, they are crucial indicators of changes in the overall climate. During the last decade many more measurements with higher accuracy have been made of temperatures in the upper layers of the ocean and in some parts of the deeper ocean too. These have shown a slow but steady temperature rise broadly consistent with the increase in warming at the ocean’s surface due to human influences, especially the release of greenhouse gases. Regarding the average surface air temperature before and since 1997, a detailed study Climate science: The cause of the pause has been published in a recent issue of Nature. A more readily available explanation (not behind a paywall) of the importance of ocean temperatures can be found at RealClimate. An obvious feature of the temperature record is the substantial year to year variability. A dominant year is 1998 with a peak value of surface air temperature 0.3C higher than in 1995 – a value that has only marginally been exceeded since. This rise is associated with the largest El Nino event ever recorded in the Pacific Ocean. These events have occurred over centuries of record and involve a large tongue of surface water, up to a few degrees C warmer than usual, extending across the whole Pacific Ocean from west to east and about 20 degrees broad in latitude. They occur every few years (three much smaller ones have occurred since 1998) and have a significant influence on the world’s weather over periods up to many months. Their presence can frequently be identified in variations within the surface temperature record. Interpretation of this record needs to take into account all known natural forcings as well as those due to greenhouse gases.

Why not ‘Wait and See’?

The increase of surface temperature or the extent of more climate extremes, even dramatic ones, does not seem very threatening to the public at large, especially when doubt is so frequently cast on the truth of the climate story as spelled out by the scientific community. So it is said “why not tackle it relatively slowly and ‘Wait and See’ what happens; if it gets a lot worse we can begin to do more.” But ‘Wait and See’ is very far away from an adequate response. What is not generally realized is that we are already committed to climate change far in excess of what we are yet experiencing. That is because of the long time it takes the oceans to warm and the very large thermal capacity they contain. Even if all greenhouse gas emissions into the atmosphere were stopped tomorrow, the earth’s surface and the upper layers of the oceans would continue to warm for many decades into the future. Sea level would continue to rise at increased rates and climate extremes such as heat waves, floods and probably droughts too would become much more frequent. We owe it to our children and our grandchildren to ensure that we reduce greenhouse gas emissions as fast as we possibly can so that as much damage as possible is avoided. Further, detailed studies by the International Energy Agency, the intergovernmental body concerned with energy futures, demonstrate clearly that cutting emissions is both affordable and will bring many co-benefits (see for instance Redrawing the Energy-Climate Map, World Energy Output Special Report 2013). Sir John Houghton (with some additional information from Dr Martin Hodson) 28 September 2013 About Sir John Houghton.