Climate Change

How hot is hot: Advanced models forecast up to 5.6°C global warming

Climate models earlier didn't account for heat trapped by clouds

By Pushp Bajaj
Published: Tuesday 28 January 2020
The research found that early-warning signals existed before global warming.

Global average temperature, previously estimated to rise between 1.4 degree Celsius C and 4.5°C from pre-industrial levels, is expected to rise further due to previously underestimated impact of clouds on the climate, a new study published in the Geophysical Research Letters journal said.

Drawing from projections by 27 advanced climate models, the study said the global average temperature would eventually increase between 1.8°C and 5.6°C. This means that the higher estimate is at least 1°C higher than previously thought.

Cloud-related feedbacks included in newer models are responsible for higher levels of predicted warming. Revised estimates pertain to a scenario wherein atmospheric concentrations of carbon dioxide (CO2) have doubled, keeping the pre-industrial baseline. 

How do clouds impact climate? 

Clouds have a dual effect on our climate — they reflect part of the sunlight that falls on them back into space, which decreases temperatures, and they trap part of the heat coming from the land/ocean surface, which increases temperatures.

“The net effect depends on the type of cloud,” George Tselioudis, senior scientist at NASA Goddard Institute for Space Studies, recently saidThe behaviour of clouds and its impact on climate varies according to the properties of the cloud itself.

For example, thick, low clouds are “strong coolers” since they reflect a large part of solar radiation (sunlight) and absorb little heat from the ocean and land, explained Tselioudis.

On the other hand, very thin, high clouds such as cirrus clouds reflect little sunlight, but they are very efficient at absorbing thermal radiation (heat), making them “strong warmers”, he added while saying that some clouds can be neutral as well.

If the new estimates are to be believed, it will be difficult to meet the Paris Agreement target of limiting the rise in global temperatures to 1.5°C, Mark Zelinka, scientist with the Lawrence Livermore National Laboratory in California and lead author of the study, told news agency AFP.

The new generation of models will guide future climate change projections and will be used in the upcoming Sixth Assessment Report (AR6) of the United Nations’ Intergovernmental Panel on Climate Change (IPCC), due to be released 2021-22.

With each passing year, there has been a rapid increase in the rate of global warming and the associated problems of fast-melting ice-caps and rapid increase in sea levels. 

How do climate models work

To understand the scale of impact of climate change, we need to know certain fundamental aspects of climate change projections. Climate models are computer programmes that analyse the relationship of land, ocean and air with the Earth’s climate. Part of the computation relies on equations, based on well-established theories, and other part involves estimates based on observed correlations.

Scientists use these models to run simulations to predict how the climate will change in the future. By default, there are inherent limitations in these models. Some relationships are simply too complex to model, and, hence, not considered in the simulations. These include some relationships of climate with wind pattern, ocean and clouds.

Such limitations make the researchers’ task of coming up with projections of future climate change difficult. One example of this can be seen in projections of sea level rise.

The IPCC’s SR15 report released in 2018 stated that a 1.5°C warmer world mean that sea levels could rise up to 0.77 metres by 2100. However, many experts believe that these estimates are far too conservative because they miss important physical processes contributing to rapid change.

“The land ice melt [in Greenland and Antarctica] alone is currently on a trajectory of 0.80 m sea level rise by 2100,” said Eric Rignot, chair for Earth System Science at the University of California in Irvine, in an interview last year.

“So it is hard to believe that somehow the system will drop to 0.26 m sea level rise when basic physics of ice sheet decay and melt dictates that the melt increases exponentially with time,” he added.

Further contribution coming from the thermal expansion of ocean water caused by increasing temperatures will add to this. The total sea level rise will therefore be much higher than the projections of IPCC’s SR15 report.

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