New Delhi, April 19 -- As Indians prepare themselves for what could be a third summer in a row of extreme heat, they might be surprised to hear their country is warming more slowly than many others. Last year, India saw its highest recorded temperature and its longest spell of heat waves. But annual mean temperatures have risen by less than 0.7degC since 1901 - about half the global average. On a world map showing how 2024 temperatures deviated from a historical baseline, many countries were coloured deep red, denoting an increase of 1degC to 2degC. But one large, pale spot stood out in the tropics. Scientists don't fully understand the actual reason; however, they have gone ahead and attributed other "cooling" factors that might be at play in India, from air pollution to irrigation. Air pollution is a leading hypothesis to explain the slow warming. Over the densely populated Indo-Gangetic Plain, which stretches across northern India from Pakistan to Bangladesh, industry, urban traffic, dust, cooking, and crop fires conspire to produce some of the world's worst air pollution - including aerosols that reflect solar radiation and may help cool the region. Globally, such pollution is thought to have partially masked warming by greenhouse gases for much of the 20th century. Conversely, global reductions in pollution may have accelerated warming in the past decade, according to a 2024 study. Another recent study found that pollution control in China raised the average temperature there by 0.1degC between 2013 and 2019. The implication is that although India badly needs to clean its air - pollution here is linked to more than 1 million deaths annually - such efforts could also accelerate warming, which the country would need to prepare for. During a recent meeting of the Indian Environment ministry with scholars of Harvard University, climate scientists made local headlines when they suggested India could expect to warm twice as fast in the next few decades. However, some scientists are cautious about the pollution hypothesis. Aerosol pollution in India includes a lot of soot particles, which absorb rather than scatter solar radiation and thus actually warm the air. It's not yet clear whether the net impact of aerosols is cooling or warming. Although satellite data suggest a net cooling, ground-based measurements are yet to confirm that. It has also been noted that regional air pollution peaks in the winter months, which have also seen the most warming. Hence, clearly, aerosols don't seem to explain the seasonal pattern. Another explanation might lie in changing winds. A couple of years back, it was found that average warming over the arid Middle East has pulled monsoon winds over the Arabian Sea northward faster - a shift that accounts for an increase in extreme rains and floods in historically dry Pakistan and northwestern India. It is now being investigated whether wind shifts in other seasons could be playing a role in the slower-than-average warming of India.
Last year was the hottest on record in India, as on Earth as a whole. But India has warmed much less than most countries on an annual basis, even as it has been hit by lethal summer heat waves. A third possible explanation is the dramatic expansion of irrigation in northern India over the past century. As water evaporates from soil or is transpired by plants, it absorbs heat from the air. In a 2016 study, researchers found that increased evapotranspiration in the US Midwest, from both irrigation and intensified farming, has cooled the hottest summer days there. A 2020 study concluded that expanded irrigation in the 20th century has dampened warming worldwide - and especially in South Asia. But the irrigation hypothesis has its critics, too. Some Indian researchers argue that studies using satellite measurements and global data sets have overestimated how much water is pumped in northern India, especially in summer, when irrigation is at its lowest, according to ground-based data, but the warming deficit has been pronounced. India's slower warming mainly reflects the country's location in the humid tropics and variability in the climate system. Pollution and irrigation might affect warming locally but at the scale of the subcontinent, they could also just be simple noise. The ocean pulling down the planetary average rate of warming provides a background for many land masses warming more quickly than average, but it's not the only reason for the differences in warming speed from place to place. Scientists know, for instance, that human-caused warming is amplified at the poles. Part of the reason for this is the albedo effect: as warming ocean and air temperatures cause polar ice to melt, it exposes the region to darker, heat-absorbing water, and less energy-reflecting white ice, further exacerbating warming. Arctic warming is also exacerbated by the region's lack of atmospheric convection - the process by which the Earth's surface heats the air closest to it, causing the warm air to rise and mix with higher air layers. That process is common in the tropics, but less common in the cold Arctic, which makes Arctic air less mixed and leads to stronger warming near its surface. The region's warming is also affected by the movement of water vapour from the tropics to the poles.
But what about locations like the Middle East, which aren't home to ice and snow and aren't located at high latitudes? Is the ocean's slow warming trend the only thing to blame for these locations' comparatively speedy temperature rise? Perhaps the answer is negative. In fact, there are several locality-specific reasons why a location might warm more quickly than others. For the Middle East, geography is a big one - the region is home to vast expanses of desert, which means it gets little cooling assistance from water evaporating from trees and other vegetation, which research has shown can slow down warming. Phenomena like localised pollution, deforestation and urbanisation, and the urban heat island effect can also cause a location to warm more quickly than average. Scientists have long known that the warming of the planet is not uniform. But rather than casting doubt on the validity of climate science, the findings that different regions of the planet warm at different rates support climate scientists' predictions, based on their understanding of the Earth system. And though regional warming trends differ, the Earth's overall temperature trend has been increasing over the last several decades, with other clues - melting glaciers, retreating sea ice, changes in the ranges of plants and animals - also painting a clear picture of warming. Hence, "global warming" essentially means Earth's average annual air temperature is rising, but not necessarily in every single location during all seasons across the globe. That's the way it is with Earth's near-surface temperature as atmospheric greenhouse gas levels climb. Temperature trends across the entire globe aren't uniform because of the diverse geography on our planet - oceans versus continents, lowlands versus mountains, forests versus deserts versus ice sheets - as well as natural climate variability.
Published by HT Digital Content Services with permission from Millennium Post.