Understanding the past of climate change can help us refine predictions for the climate of tomorrow, says IPCC lead author.
The UN released a benchmark report on the state of the climate on Monday, giving a stark warning of what the future holds if humans do not curb their carbon emissions and fast.
According to the report by the Intergovernmental Panel on Climate Change (IPCC), without quick action, the planet will continue to warm up, leading to worse and more frequent droughts, floods, heat waves and other extreme weather events. Ice sheets will keep retreating and sea levels rising, threatening coastal communities worldwide.
What’s more, the report highlights an unprecedented warming rate in at least 2000 years. But how are scientists able to make such assertions about a time so far back when there were no thermometers or satellites and how is it useful for predicting the future of climate change?
The stark contrasts of history
To come up with the most accurate account possible for what could happen tomorrow, scientists rely on different methods, such as observing current changes or studying past ones through satellite imagery or weather stations. But satellite records only go a couple of decades back and other instrument records are no more than 100 years old.
By studying the geological records of biological and chemical materials buried deep under the ocean or rigidly frozen within glaciers, researchers can know what the climate was like several millions of years ago.
“Knowing what happened in the past, puts future changes into context,” climate science professor Dan Lunt from the University of Bristol and one of the lead authors of the released climate report told Geneva Solutions. “It allows us to realise how unprecedented changes are happening now and it allows us to separate actual changes from human-induced changes as well.”
One striking contrast between then and now highlighted by the report is that levels of carbon dioxide in the atmosphere are at their highest since at least two million years.
“It means we've compressed two million years of natural carbon dioxide change into just a few decades and that's a powerful statement for putting current changes into context,” Lunt observed.
Under the highest emissions scenarios, CO2 levels could break all records in the last 60 million years.
Lessons from the past
Understanding past changes is not only useful for making sobering statements. It actually helps forecast what the impacts of those changes can be in the future.
Three million years ago, for example, global temperatures were 2.5 to 4ºC above current temperatures – one of the scenarios that could play out by the end of this century. At that point, sea levels were about 10 metres higher than where they are now.
“That doesn't mean that sea levels will be 10 metres higher by the end of the century because sea level can take a long time to adjust to the warming. What it means is that with that amount of warming we're setting ourselves on a path where there might be 10 or more metres of sea-level rise and that could mean completely wiping out whole coastal communities,” Lunt said.
Rising seas are one of the effects of climate change that once it has been set in motion, is irreversible for hundreds to thousands of years, according to the report.
That scientific knowledge can also be used to test current climate models devised to predict future changes by comparing them to what actually happened in periods of extreme conditions such as high levels of carbon dioxide, according to geological data, Lunt explained.
“Going back to the past allows us to test the models under conditions that might be similar to those that they're being used to predict for the future,” he said.
For the IPCC, which relies on a great number of climate models to come up with different scenarios, testing their accuracy can help narrow down the range of uncertainty.
“One of the things we found in this report, for example, is that the models that show the least amount of warming aren't very good actually at predicting the past, but also the models that predict the most amount of warming in the future also aren't so good at predicting the past. We can narrow that range of uncertainty by saying that the most extreme models are probably too warm and the least extreme models are probably too cold,” he explained.
When it hits close to home
As climate models have improved, scientists’ ability to make predictions on a finer scale has also grown. The report highlights that climate change is affecting all parts of the world, but the consequences are not the same everywhere. Regions in high latitudes will likely see more rainfall, while it is expected to drop in large parts of the subtropics.
“The way that these models work is they break the world down into a number of individual boxes. The models can only tell you about what’s happening in the size of one of these boxes and over time these boxes have got smaller and smaller,” Lunt said.
“Whereas in the first IPCC report we might have been able to say something about what was happening in central Europe, now we can actually say something about what's happening in a particular part of Switzerland,” he added, citing the example of the Swiss Alps.
IPCC’s assessment shows that the Swiss Alps are warming up and that temperatures rise faster in higher altitudes. Changes in snowmelt will affect stream flow into valleys, potentially depleting water reserves in the summer. The tourism industry of ski stations, a major source of income for Switzerland, could also be heavily affected.
Lunt stressed that the overall picture of climate change remains largely unchanged, but the increase in certainty and the new regional focus are what the report will bring to the table, particularly with the climate summit Cop26 less than three months away.
“For policymakers that can be very important because many are focused on their local issues and we can now tell them more about what those might be,” said Lunt.
Such data about the local implications of climate change could make the difference in encouraging governments to make strong commitments to tackle the climate crisis.