The Intergovernmental Panel on Climate Change (IPCC) keeps asking the question: what will the future climate be like under several different emission scenarios?
In a regular series of reports from as far back as 1990, scientists have been giving the same answer: the more greenhouse gases we emit, the warmer the world will get. The repetition is getting boring. The only difference over the past 30 years is that as the data builds up, as the science progresses, as the models get better and computers get more powerful, there is more confidence and more detail in that answer.
What was “likely” just a few years ago is now “virtually certain”, to the point where UN secretary general António Guterres called the latest AR6 report “A code red for humanity”. It found that human activities have unequivocally warmed the Earth’s climate across the entire planet, and that these changes are widespread, rapid, and intensifying.
Advances in both climate science and supercomputing have enabled the continuous improvement of climate models. In 1990 climate simulations could afford only coarse representations of the atmosphere, without the ability to factor in dynamic ocean or land processes.
In contrast, the latest report assesses large ensembles of high-resolution models that couple together all the components of the real climate: atmosphere, ocean, land/vegetation, and sea ice. Today’s climate models synthesise and represent numerically all the latest scientific knowledge about radiation, cloud microphysics, turbulence and other processes that together determine climate.
Consequently, our ability to predict the climate response to any particular future emissions scenario (or “forcing”) has become much more robust. The main question now is, what will those emissions actually be?
For any specific trajectory of future greenhouse gas emissions, climate models project what the climate response would be. We speak of climate “projections” instead of “forecasts” because the climate over the next few decades depends to some extent on human-controlled emissions, and we cannot be sure how humans will choose to exercise that control.
Ireland’s input
One way that Ireland contributes to the IPCC reports is through its participation in a consortium of several northern European countries that develop and run one of the 50 or so global climate models that feed into the IPCC assessments.
The Irish Centre for High-End Computing (ICHEC) has conducted many centuries worth of global climate simulations with the 'EC-Earth' climate model. As an "earth system model", it represents all of the most relevant physical processes that operate within and between the atmosphere, the ocean, the land surface, and sea ice. Simulations range from the historical past, so that model performance can be compared with real climate records, out to the end of the 21st century. This work is supported by the EPA, Met Éireann and the Marine Institute.
Over 50 different global climate models have been developed by different research centres around the world, including Ireland, via ICHEC and Met Éireann. Like Formula-1 cars, they may share some component parts, but each is also different in its own way.
Their real strength, however, is when they are collectively organised to each run the same simulation experiments. Insofar as the particular design of each model permits, they all use the same initial configuration, the same boundary conditions, and the same future emission scenarios. Collectively, they generate ensembles of many thousands of sample climates for each scenario.
That number is big enough to generate high confidence that something close to the full range of possible climates is being captured. Voltaire’s Dr Pangloss would be impressed. Multi-model ensembles like this are great examples of the whole being greater than the sum of its parts.
These coordinated climate simulation projects also reflect how climate research has become “big science”, with no one person or even one institute able to do it all. ICHEC’s simulations alone have consumed tens of millions of computer core-hours and the resulting datasets occupy over 1.5 petabytes (equivalent to the storage capacity of 1500 well-endowed modern laptops).
The potential for more extreme events
Pretty much all models from 30 years ago until now predict that the world will continue to get warmer if greenhouse gas emissions continue on their current trajectory. A mean warming of 2 or 3 degrees may not seem like much, given that temperatures can vary by a lot more than that just from day to day. However, even that level of warming is likely to lead to widespread and even dramatic changes in ice cover (especially in the Arctic), and in the natural world of plants and animals.
Moreover, a changing climate tends to shift the full range of expected weather, not just the mean. Even more dramatic future changes are projected for the extremes, or “tails”, of the frequency distributions of each sample climate. Thus, a rainfall event so heavy that it only occurred once every 100 years or so in the past, may occur every 20 years in the future. Similarly, long deep freezes (as in the winter of 2010-11) may become even rarer in a future warmer world.
What does this mean for Ireland?
ICHEC scientists have supplemented the global projections with a further series of regional simulations (each nested within a larger global simulation) to provide a set of ultra-high resolution future climate projections for Ireland. These provide a wealth of detail about the likely features of Ireland's future climate. Selected findings from this study are that by the middle of this century (2041-2060) we should expect:
– Temperatures to increase by 1.0-1.6 degrees (relative to 1981-2000) with the largest increases in the east;
– More frequent summer heatwave events, especially in the south;
– Approximately 50 per cent fewer frost and ice days;
– More variable precipitation, with more dry periods and more heavy precipitation events;
– A decrease in snowfall of 50 per cent or more;
– Weaker surface winds, along with decreased energy content of the 120 metre (turbine height) wind in all seasons;
– Annual growing and grazing seasons to lengthen by 12-16 per cent. Similarly, crop heat units and growing degree days for a range of crops (and pests) are likely to increase substantially.
Supercomputing capability has increased by over seven orders of magnitude, or a factor of 10 million, over the past 30 years. This has enabled advances in climate science which would have been unthinkable even as recently as 1990. Given the computing resources now available and other scientific advances, model simulations can provide very precise indicators of our climate.
The level of detail and consistency achieved gives confidence in these projections and allows an ever more persuasive evidence-based consensus to emerge that humans are forcing rapid climate change in well-understood ways.
How to respond to that consensus now is a matter primarily for governments (since they can have the most impact) as well as for individuals.
Dr Enda O’Brien and Dr Paul Nolan are based in the Irish Centre for High-End Computing in Galway