Climate Science - What does the Future Hold for our Climate?
So far in our climate science series, we have introduced the basics of climate science, shown how humans are already impacting the climate, explained tipping points and feedback loops, and given a brief history of how we got to this point. In Part 5 of our series we will look to the future, how do we predict what our climate will look like and what do those predictions indicate?
How do we predict the future?
The simplest way to predict the future is to look to the past and take note of trends. The last seven years have been the warmest seven years on record, and as the director of GISS puts it:
“With these trends, and as the human impact on the climate increases, we have to expect that records will continue to be broken.”
Trends can show what is likely to happen if things stay the same, but to get more precise predictions and to see what might happen if things do change, we have to turn to climate models.
Climate models are essentially computer simulations of the Earth’s climate system, they break the atmosphere down into a grid of smaller and smaller 3D “parcels” and map climatic processes such as ocean circulation and melting glaciers. The processes use complex mathematical equations to simulate the impact of changing parameters from one parcel on to it’s neighbours. When the interactions between all the parcels that make up the entire atmosphere are calculated, the model can give us predictions of how the earth will respond to certain scenarios.
For instance, you could “plug in” a steady rise in CO2 emissions and the model would run all the calculations to show you how the atmosphere would respond under that scenario.
To test the accuracy of their models scientists can run them using data from the past and check if their results match up with historical records, once satisfied they can replicate the past climate accurately they are run into the future to try and see how our climate might behave. Due to the number of parcels required for a detailed model, not to mention the complexity of their interactions, the models need to be run in state-of-the-art supercomputers, such as this one used by the UK Met Office.
What do the models predict?
Clearly there is not one single pre-determined future outcome for our climate. The climate of 2100 depends on what we do between now and then. If we stop CO2 emissions tomorrow the future climate will look very different compared to a future where emissions continue to rise at their current rate.
Efbrazil, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
There are a lot of different climate models, and an infinite number of potential scenarios, so for simplicity we will focus on those scenarios used by the Intergovernmental Panel on Climate Change (IPCC). The IPCC based its Fifth Assessment Report on four “Representative Concentration Pathways” (RCPs) mapping four different projections of GHG emissions, going from a scenario where emissions continue to rise (RCP8.5) down to one where emissions fall (RCP2.6):
The IPCC projections of the increase in global temperature based on these scenarios show a range of potential outcomes. Some meet the Paris Agreement aims of 1.5deg C, some do not. Unsurprisingly, the key takeaway is that the faster we reduce CO2 emissions the more likely we are to stay under 1.5degC.
IPCC. Special Report. 2018. Figure SPM.1. https://www.ipcc.ch/sr15/chapter/spm/
What will be the outcome of this temperature rise?
Any temperature rise will have an impact, and we have shown that climate change is already happening just on the temperature rise we have had since pre-industrial times. But for more detail, we have looked to a 2018 report by the IPCC called “Long-term Climate Change: Projections, Commitments and Irreversibility” which “assesses long-term projections of climate change for the end of the 21st century and beyond”.
The IPCC use various terms to refer to future scenarios in this, and other, reports. A helpful Glossary is provided here:
Its findings are stark, and we have summarised them below, paraphrasing occasionally when it delves into technical language but largely lifting directly from the executive summary:
Projections of Temperature Change
Temperature change will not be regionally uniform.
It is virtually certain that, in most places, there will be more hot and fewer cold temperature extremes as global mean temperatures increase.
Global mean temperatures will continue to rise over the 21st century if greenhouse gas (GHG) emissions continue unabated. By 2100 they predict the temperature rise will be:
0.3°C to 1.7°C (RCP2.6)
1.1°C to 2.6°C (RCP4.5)
1.4°C to 3.1°C (RCP6.0)
2.6°C to 4.8°C (RCP8.5)
Which graphically looks like this:
Changes in Atmospheric Circulation
Mean sea level pressure is projected to decrease in high latitudes and increase in the mid-latitudes as global temperatures rise.
Changes in the Water Cycle
It is virtually certain that, in the long term, global precipitation will increase with increased global mean surface temperature.
Changes in average precipitation in a warmer world will exhibit substantial spatial variation. Some regions will experience increases, other regions will experience decreases and yet others will not experience significant changes at all.
Annual surface evaporation is projected to increase as global temperatures rise over most of the ocean and is projected to change over land following a similar pattern as precipitation.
Changes in Cryosphere (Ice landmasses)
It is very likely that the Arctic sea ice cover will continue shrinking and thinning year-round in the course of the 21st century as global mean surface temperature rises. At the same time, in the Antarctic, a decrease in sea ice extent and volume is expected, but with low confidence.
It is very likely that Northern Hemisphere snow cover will reduce as global temperatures rise over the coming century. A retreat of permafrost extent with rising global temperatures is virtually certain.
Changes in the Ocean
The global ocean will warm in all RCP scenarios.
Ocean warming will continue for centuries, even if GHG emissions are decreased or concentrations kept constant.
It is very likely that the Atlantic Meridional Overturning Circulation (AMOC) will weaken over the 21st century but it is very unlikely that the AMOC will undergo an abrupt transition or collapse in the 21st century.
Long-term Climate Change, Commitment and Irreversibility
Global temperature equilibrium would be reached only after centuries to millennia if (emissions) were stabilized.
A large fraction of climate change is largely irreversible on human time scales, unless net anthropogenic CO2 emissions were strongly negative over a sustained period.
Climate Stabilization
The principal driver of long-term warming is total emissions of CO2 and the two quantities are approximately linearly related. Half of the CO2 budget that we can “allow” to be emitted to keep the temperature rise below 2degC was used up by 2011.
Some aspects of climate will continue to change even if temperatures are stabilized.
Abrupt Change
Several components or phenomena in the climate system could potentially exhibit abrupt or nonlinear changes, and some are known to have done so in the past. Examples include the AMOC, Arctic sea ice, the Greenland ice sheet, the Amazon forest and monsoonal circulations. For some events, there is information on potential consequences, but in general there is low confidence and little consensus on the likelihood of such events over the 21st century.
What can we take from all this?
It makes for sobering reading, and it is worth noting that according to the IPCC ALL of the listed impacts are made much worse if we follow the pathways that reduce GHGs the least (RCP 6.0 and RCP8.5). This just underscores the importance of reducing emissions, and reducing them fast. But it is not all doom and gloom, there is still a chance we can keep temperature rises below levels that would induce devastating consequences. With co-ordinated and urgent global action we can still avoid some of the worst impacts.
The various scenarios show us that the more we can reduce emissions the less devastating climate change will be, but there is no time to lose!
Find out more:
IPCC Special Report on 1.5degC Warming. https://www.ipcc.ch/
Fourth National Climate Assessment (NCA4) of the U.S. Global Change Research Program (USGCRP). Chapter 4: Climate Models, Scenarios, and Projections
NASA. Are Climate Models getting future projections right? https://climate.nasa.gov/news/
UK Met Office - Future Models. https://www.metoffice.gov.uk/
NOAA Explanation of climate models. https://www.climate.gov/maps-data/
Carbon Brief. Q&A How do Climate Models Work? https://www.carbonbrief.org/