The annual mean temperature of the world is known to have increased by 1.1 degree Celsius from the average of the 1850-1900 period. But this increase, as can be expected, is not uniform. It varies in different regions and also at different times of the year. This single number denoting global temperature increase, very effective for communicating the dangers of climate change, is built on top of several layers of averages.
Temperature rise over land is much higher than over oceans. Over land, the annual mean temperatures have risen by as much as 1.59 degree Celsius since preindustrial times, according to the latest report of the Intergovernmental Panel on Climate Change. Oceans, in contrast, have warmed by about 0.88 degree Celsius.
The warming trends over the Indian region are very different. An assessment of climate change over the Indian subcontinent, published by the Ministry of Earth Sciences in 2020, said annual mean temperatures had risen by 0.7 degree Celsius from 1900. This is significantly lower than the 1.59 degree Celsius rise for land temperatures across the world. It could give the impression that the problem of climate change over India was not as acute as other parts of the world. But that is not entirely accurate.
Why is warming over India lower?
The relatively lower rise in temperatures over India is not a surprise. Also, India is not a special case. The increase in temperatures is known to be more prominent in the higher altitudes, near the polar regions, than near the equator. This is attributable to a complex set of atmospheric phenomena, including heat transfers from the tropics to the poles through prevailing systems of air circulation. India happens to be in the tropical region, quite close to the equator.
10-year moving average mean maximum temperature.
10-year moving average mean minimum temperature.
A substantial part of the difference between the temperature rise over India and that over the entire world can be explained by understanding what the different numbers represent.
The planet as a whole has warmed by 1.1 degree Celsius compared with preindustrial times. But, as mentioned earlier, this is just the average. Different regions have seen very different levels of warming. The polar regions, particularly the Arctic, have seen significantly greater warming. The IPCC report says the Arctic region has warmed at least twice as much as the world average. Its current annual mean temperatures are about 2 degrees Celsius higher than pre-industrial times. Some other studies suggest the Arctic could be warming even faster.
Again, this happens because of a variety of reasons, including the processes mentioned earlier. Another prominent cause is what is known as the albedo effect, or how much sunlight a surface reflects. The ice cover in the Arctic is melting, because of which more land or water is getting exposed to the Sun. Ice traps the least amount of heat and reflects most of the solar radiation when compared with land or water. More recent research suggests that the higher warming in the polar region could be attributed to a host of factors, including the albedo effect, changes in clouds, water vapour and atmospheric temperatures.
The warming in the polar regions account for a substantial part of the 1.1 degree Celsius temperature rise over the entire globe.
Higher warming over land than oceans
However, the 0.7 degree Celsius temperature rise over India has to be compared with the warming seen over land areas, not the entire planet. As mentioned, land areas have become warmer by 1.59 degree Celsius.
Land areas have a tendency to get heated faster, and by a larger amount, than oceans. Daily and seasonal variations in heating over land and oceans are usually explained in terms of their different heat capacities. Oceans have a higher capacity to cool themselves down through the process of evaporation. The warmer water evaporates, leaving the rest of the ocean relatively cooler.
However, longer-term enhanced heating trends over land have to be attributed to other, more complicated, physical processes involving land-ocean-atmospheric interactions.
Impact of aerosols
Aerosols refer to all kinds of particles suspended in the atmosphere. These particles have the potential to affect the local temperature in multiple ways. Many of these scatter sunlight back, so that lesser heat is absorbed by the land. Aerosols also affect cloud formation. Clouds, in turn, have an impact on how much sunlight is reflected or absorbed.
Aerosol concentration over the Indian region is quite high, due to natural as well as man-made reasons. Due to its location in the tropics and the arid climate, India is no stranger to dust. But it also happens to be experiencing heavy pollution right now. Emissions from vehicles, industries, construction, and other activities add a lot of aerosols in the Indian region. A reduction in warming could be an unintended but positive side-effect.
Tropical location key
A major part of India’s relatively lesser warming can be attributed to its location in the lower latitudes. As Bala Govindasamy, professor at the Centre for Atmospheric and Oceanic Sciences at the Indian Institute of Science in Bengaluru, said, the fact that higher latitudes experience greater warming is now fairly well established in science.
A majority of the global landmass is concentrated in the northern latitudes. In the tropics and along the equator, it is mostly oceans. Land areas are also prone to faster, and greater, heating. Because of both these reasons — that lands heat up more, and most of the land is located in northern latitudes — the average warming over global land areas has become more pronounced. For a country like India, located in the tropics, the deviation in temperature rise from the global average is not surprising.
Aerosols could also be playing a role, but the extent of the impact is not very clear right now. As M Rajeevan, former Secretary in the Ministry of Earth Sciences, put it, aerosols have the potential to avoid 0.1 to 0.2 degree Celsius of warming over the Indian region. Tragically, this would mean that as we clean up our air, temperature rise could be faster.
Incidentally, while the maximum temperatures over India have shown a significant increase since 1900, the rise in minimum temperatures has not been much. The rise in annual mean temperatures, therefore, has been mainly due to the increases seen in the maximum temperatures. The reasons for this are not very well understood. The climate system operates at a global level, and modelling it on regional scales introduces a lot of uncertainties.
The green hills of Vetal Tekdi, Pune, turned brown in March as temperatures rose. (Express photo: Arul Horizon) 
10-year moving average mean maximum temperature.
10-year moving average mean minimum temperature.
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