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Carbon Dioxide more efficient at warming the earth than sunlight, show IISc scientists

Climate is one of the most complex and extensively studied systems and yet one of the most difficult to predict. It has been known for some time now that greenhouse gases (GHGs), especially Carbon Dioxide (CO2), is a major culprit for the increasing temperatures in the recent decades and that sunlight or solar radiation can warm up the atmosphere but not as much as the GHGs do. Scientists at the Indian Institute of Science, Bangalore, have now come up with climate models that offer explanations for these differing inefficiencies in the warming caused by two of the main perpetrators of climate change.

The climate of a region could be considered to be highly variable since it depends on varying external factors such as solar input, greenhouse gases and aerosols. One of the main components in calculating climate change is Radiative Forcing. It is the difference in the amount of sunlight absorbed by the earth and the energy leaving the earth. A positive forcing denotes that more energy is being absorbed than what is released into space. A positive forcing warms the climate and is usually instigated by changes in sunlight, greenhouse gases like carbon dioxide, methane and black carbon aerosols in the atmosphere and negative forcing caused by sulphate aerosols, volcanic eruptions, glacial cover etc.

For long, scientists have known that solar forcing (the radiative forcing caused by direct sunlight) isn’t as efficient as the forcing caused by the GHGs. The reason for the inconsistency however, was never fully understood until now. An international team of scientists from IISc, India, Zhejiang University, China and Carnegie Institute for Science, USA has proposed climate models and simulations to explain the inconsistencies.

“This is a very tough problem. Although there is a lot of literature on the lower efficacy of sunlight, there has been none to explain why solar forcing is less efficient compared to CO2 forcing. Scientists have known for long that solar forcing isn’t very efficient. Our work explains as to why it is inefficient”, says Prof. Govindasamy Bala from the Center for Atmospheric and Oceanic Sciences (CAOS), IISc.

The team split into two groups and each used a different climate model to study the effects of forcing. “The two groups used different climate models, but agreed on the results. This makes our study a robust one and the confidence in our explanation is high”, says Prof. Bala. “We have found that solar forcing is only about 80% as effective as CO2 forcing. We offer explanations for the differing efficiencies”, he adds.

The reason could be the difference in the way solar and CO2 forcing work. “The heating patterns of solar heating and CO2 absorption are vastly different. CO2 forcing cools the stratosphere while it heats up the troposphere. Solar forcing, on the other hand, heats both stratosphere and the troposphere. Various other factors such as the rapid response of clouds and accelerated warming of land surface compared to oceans also play roles in determining the amount of climate change for a given radiative forcing” explains Prof. Bala.

The current study has strong implications in the field of climate change. It allows us to make improved predictions of the climate change for changes in the amount of sunlight on long time scales. The study could also be used as a tool to interpret past climatic changes, such as during the Maunder Minimum (a period between 1645 and 1715 when sunspots became extremely rare and solar irradiance was reduced). It also increases the precision of our prediction of climatic response following events like volcanic eruptions and changes in glacial cycles. Thus, the study forms a handbook to better understand the factors that affect our climate.

About the authors:

Prof G. Bala is a professor at Center for Atmospheric and Oceanic Sciences and is associated with Divecha Center for Climate Change and the Inter-disciplinary Center for Water Research at IISc.


Angshuman Modak is a PhD student at the Center for Atmospheric and Oceanic Sciences at IISc.

Long Cao is a Professor at School of Earth Sciences at Zheijiang University in Zhejiang, China

Ken Caldeira is a Professor at the Carnegie Institute of Science California, USA.

About the Paper:

The Paper is titled “Why must a solar forcing be larger than a CO2 forcing to cause the same global mean surface temperature change” and was published in the Environmental Research Letters journal in April 2016.