New Delhi: Global warming is shrinking glaciers faster than thought: Associated Press, April 2019. Oceans are warming faster than predicted: Scientific American, January 2019. Antarctica’s ice sheets may melt faster than we thought, accelerating sea level rise: Quartz, January 2019.
Recent climate change predictions have been dire. But turns out, all these doomsday forecasts may have been less dire than the current situation warrants.
The United Nations Intergovernmental Panel on Climate Change (IPCC) said in a report late last year that limiting warming to 1.5°C above pre-industrial levels this century would result in vast benefits as compared to a situation where the world pursued 2°C as the goal.
Stating as much, the panel listed the advantage this feat would entail for different parameters used to detect climate change: For example, it said limiting warming to 1.5°C would restrict the rise in sea level to 0.26-0.77 m by 2100, which would be at least 0.1 m lower than at a warming of 2°C, providing humans a longer window to adapt.
Driving the conclusion were complicated computer simulations that employ climate models to paint a possible forecast for the coming years.
The computer simulations used to make projections are absolutely critical tools in exploring the range of climate impacts in hypothetical future scenarios.
However, scientists say that they may be inadequate to adjudge the true intensity of climate change. In simple words, we may have undermined the disaster awaiting us and pretty much dropped the ball on staving off some of the worst consequences of climate change.
The IPCC, which has published several reports on the causes, impact, and solutions of climate change, has emerged as the global authority on the subject.
Its reports are often cited in scientific literature and inspire extensive media reportage.
One key role of the IPCC is to make predictions for climate change impact, including on global average temperature, sea level, extreme weather events, etc., under different future greenhouse-gas-emission scenarios. These predictions are meant to guide governments around the world to devise policies and strategies to counter climate change.
These forecasts, as stated earlier, are based on studies employing computer simulations based on a variety of climate models.
“Climate models are computer programmes that contain mathematical representations of many of the processes and relationships that exist in the Earth’s climate system,” Jennifer Francis, a senior scientist at Woods Hole Research Centre, US, explained to ThePrint in an email.
“Some parts are equations based on known physical, chemical, and biological relationships, and others are estimated based on observed correlations,” Francis added.
Speaking to ThePrint, Sushil K. Dash, a climate modelling expert and professor at the Centre for Atmospheric Sciences, IIT-Delhi, explained how these models are used to make long-term predictions.
“Simulations are run for a longer time, 100 years or more in the future, under different greenhouse gas emissions scenarios to obtain values for future climate conditions,” he said.
However, there is increasing concern in the scientific community that the current models may be underestimating the problem at hand.
According to the aforementioned IPCC report — titled the Special Report on Global Warming of 1.5 degrees Celsius — model-based projections of global mean sea level rise (relative to 1986–2005) suggest “an indicative range of 0.26 to 0.77 m by 2100 for 1.5°C of global warming, 0.1 m (0.04–0.16 m) less than for a global warming of 2°C”.
The IPCC has been rather complacent in its projections of sea level rise. Until 2007, the IPCC did not include the contribution from melting of the Greenland and Antarctic ice sheets.
Every year, the ice sheets lose some mass mainly through surface melting and run-off in summer, and through solid ice discharge from the glaciers. Then they gain some mass in the winter when fresh snow falls and forms yet another layer of coating on the ice sheet. Ideally, this maintains a balance of melting ice and fresh snow.
However, as our climate is changing, this balance is shifting, and both the Greenland as well as the Antarctic ice sheets are losing more mass than they are gaining, which is raising the global sea level.
A rise in sea level is one of the biggest challenges posed by climate change, as it puts some low-lying countries at the risk of submergence.
The current IPCC projections do consider the melting of the Greenland and Antarctic ice sheets but fail to take into account the observed accelerating rate of the melting and its contribution to sea level rise.
According to Eric Rignot, the chair for Earth System Science at the University of California Irvine, US, and an expert on ice sheet and glacial melting mechanisms, based on the current trend of ice loss from the Greenland and Antarctic ice sheets, sea level rise will be about 0.80 m by 2100, which is higher than the maximum rise suggested by the IPCC if warming is restricted to 1.5°C.
“…It is hard to believe that somehow the system will drop to 0.26 m sea level rise [the lower level of the IPCC estimate] when basic physics of ice sheet decay and melt dictates that the melt increases exponentially with time,” he told ThePrint.
“As more time passes without a major effort to curb our carbon emissions, and the planet continues warming, we are committing ourselves to higher sea level rise in the long run,” Rignot said, “Not just a metre, but multiple metres.”
Simulations vs observations
This discrepancy between the model projections and current trends is due to a combination of two factors: The limitations of the models and lack of empirical data to feed into the models as parameters.
Some key relationships between wind patterns, ocean currents and their impact on the ice sheet along the ice-ocean boundary, which are accelerating the melting, are simply too complex to model and, hence, not considered in computer simulations.
Moreover, when there are physical relationships that can be effectively modelled into mathematical equations, scientists still need empirical data to obtain the parameters that go into the models.
For things like ocean temperature and topology of the sea floor near the ice sheet, there just aren’t enough direct observations available to guide the model simulations.
“In reality, winds can push warm ocean water along troughs in the sea floor that have been carved by former advances of the glaciers, [accelerating the melting],” said Rignot. Such mechanisms are often omitted in model simulations.
‘A conservative estimate’
Scientists have also taken exception to another projection made in the 2018 IPCC report, this one concerning Arctic sea ice: “With 1.5°C of global warming, one sea ice-free Arctic summer is projected per century. This likelihood is increased to at least one per decade with 2°C global warming.”
According to Francis, who has studied Arctic sea ice extensively, “this is a very conservative estimate given the reality of current conditions”.
A study published in 2013 that sought to estimate when the summer Arctic will be nearly sea ice-free used three different approaches.
The first approach, based on an extrapolation of observed sea ice volume data, suggests it could happen as early as 2020. The second method, which assumes several anomalous extreme weather events leading to rapid ice loss, suggests 2030±10 years.
The third used computer model projections that estimated the first sea ice-free Arctic summer will come in 2040 or later. The IPCC projections are even more conservative.
The reason why there is such a wide range of predictions is that our climate system is incredibly complex, and we do not yet have a complete understanding of how exactly the different atmospheric, oceanic and land-based processes interact with one another. This makes modelling these changes an extremely challenging task.
The IPCC itself acknowledges this complexity. A side note in the 2018 report states that “climate models do not include all existing Earth system feedbacks and may therefore underestimate both rates and extents of changes”.
“Temperature reconstructions of past warm intervals suggest that current climate models underestimate regional warming at high latitudes (polar amplification) and long-term (multi-millennial) global warming,” it adds.
Sea level rise and Arctic sea ice are only two examples of climate change impacts with global consequences.
How scientists draw future projections for these and other impacts will determine the decisions and policies to be adopted to mitigate and adapt to climate change. Solely relying on model projections made in IPCC reports as the absolute truth will likely keep climate change a step ahead of humans, always.
“There should be constant back and forth interaction between climate modellers, domain experts, bureaucrats, and policymakers, preferably at the regional level, in order to make effective policies to deal with future climate change impacts,” said Dash.
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