The latest report of Working Group I of the Intergovernmental Panel on Climate Change (IPCC) combines new and better observations with improved models to provide a clearer and more definite picture of humanity’s influence on the climate system, and the paths we can choose for the future.
The atmosphere continues to warm, with the last three decades each being warmer than the one before, and all three being warmer than any other decade since reliable observations began in the 1850s. Warming is seen almost everywhere on the Earth’s surface, and many observed changes are unprecedented on scales of decades to millennia. We also now have much better records of ocean heat content and of the mass balance of major ice sheets, allowing the sea level ‘budget’ to be closed. This means that the observed total rate of global mean sea level rise is equal (to within measurement uncertainty) to the sum of the various measured and calculated components (thermal expansion, loss of ice mass, ground water change). This gives greater confidence in future projections.
The final chapter (Chapter 14) of the Report, which I was involved in writing, synthesises many of the themes from earlier chapters (on past and observed changes, Earth system processes and climate model projections), drawing them together under two broad headings: climate phenomena and regional climate change. The ‘phenomena’ are the large-scale patterns of variability that have a strong influence over regional climate on seasonal-interannual scales, such as the El Niño-Southern Oscillation (ENSO), the Southern Annular Mode (SAM), the North Atlantic Oscillation (NAO), the various monsoon circulations, and so on.
A substantial fraction of Chapter 14 is taken up with tropical phenomena, since it is the heating of the tropics, and the way heat is radiated from the tropics towards higher latitudes that drives most of the global circulation of the atmosphere and oceans. Key findings for tropical regions include:
- On average, monsoon rainfalls are likely to become more intense and the monsoon systems are likely to cover a greater area than at present. This is despite an expected weakening of tropical circulations – increased atmospheric moisture content more than outweighs this. Monsoon seasons are likely to get longer, mostly through a delay in monsoon withdrawal dates.
- In most tropical regions (and for much of the rest of the globe), the rule of thumb is that ‘wet gets wetter’ and ‘dry gets drier’. However, it also appears that in the tropics, the ‘warmer get wetter’, i.e. those regions of the wet tropics that warm the fastest are likely to see the largest rainfall increases.
- The ENSO cycle will remain the dominant pattern of interannual climate variability globally into the future. There is no strong indication that the form of ENSO events will change in future, but rainfall variability associated with ENSO is likely to increase.
- Tropical cyclone numbers are unlikely to increase, but cyclone maximum intensity is likely to increase in the global average, meaning increased maximum precipitation and winds.
Outside the tropics, the main features of note are the annular modes (SAM and NAM/NAO) and the associated storm tracks, blocking anticyclones, and so on. As for tropical cyclones, the frequency of mid-latitude cyclones are likely to decrease slightly or stay about the same as present. Extremes of cyclone-related precipitation are likely to increase, and the Southern Hemisphere storm tracks are likely to migrate slightly towards the Poles.
The SAM has exhibited a trend towards its positive polarity over the past four decades, largely driven by ozone depletion, with a contribution for greenhouse gas increase. As the ozone ‘hole’ recovers through the next 50-60 years, the SAM trend is likely to cease in summer, but there is likely to be a continued weak positive trend in winter. In the annual mean, this fits with a slight poleward contraction of the storm tracks and associated jet stream.
In the Northern Hemisphere, the NAO and the Northern Annular Mode (NAM) exhibit considerable variability comparable in magnitude to anthropogenically-forced trends. Hence, while the NAO and NAM are likely to exhibit a small trend towards their positive polarity, there will continue to be considerable variability on all time scales.
Regional climate changes
For the purposes of describing regional climate change, the globe is divided into 18 regions as used in the IPCC Special Report on Extremes. Throughout the text of Chapter 14, regions and phenomena are cross-referenced, to paint a picture of how different components of natural variability affect different regions, and how they are changing in response to a warming planet.
Beyond the regional discussion in Chapter 14, an innovation for the new report is the production of an Atlas of global and regional changes based on output from the CMIP5 suite of General Circulation Models. This appears as an Annex to the main Report and illustrates average climate changes in the different regions (and globally). The published Atlas uses RCP4.5 for all its results, but other scenario results will be available on-line once the full report is published later this month (January 2014).
Many expected changes are encapsulated in the ideas that wet regions and seasons get wetter and dry get drier, and that land surfaces will warm faster than ocean surfaces. For many regions of the globe, climate change patterns in the 5th Assessment Report are broadly similar to those from the 4th Assessment Report.
Below are predictions based on the RCP4.5 emissions scenario, whereby total radiative forcing is stabilized before 2100 by employment of a range of technologies and strategies for reducing greenhouse gas emissions. The maps of temperature and precipitation change are averages based on 39 climate model simulations. The colours represent the predicted change from present (1986-2005 average) as follows:
- Warmer in both summer and winter
- More warming will occur in northern Europe, towards the poles, and further from the North Atlantic
- Russia and Finland will experience more warming in the winter
- UK, central and southern Europe will experience more warming in the summer, likely by 1.5-3°C in 30-50 years
- Southern Europe will be drier in summer and winter, while UK and central Europe will likely be wetter in winter but drier summer
- Warmer summer and winter
- North-west will experience more warming in the winter and will likely increase by at least 5°C in about 30-50 years
- Central and east Canada will experience more warming in the winter, while central and east coast US will warm more in the summer months
- The east coast will likely see more rain, while central-south US will likely be drier in both summer and winter
- Warmer in summer and winter
- China, Korea and Japan will likely warm by 1.5-2°C in 30-50 years, with high altitude Tibet warming by 3°C
- Large increases in rain are expected across most of Asia in summer months, but Japan could be drier in winter
- Warmer in summer and winter
- Australia will likely be drier by 10-40% in winter
- New Zealand will be generally wetter, up to 20% in the western South Island