The thermometer at a weather station in Delhi doesn't know it's part of a giant ear. Every day it records the temperature, and after forty years of such records, you can hear the breathing of an entire subcontinent. The India Meteorological Department divided the country's land into one-degree squares and took readings from 1970 to 2009. The result is a dense grid that no one had before.
From these data, the authors compiled annual averages in three ways: for the whole country at once, for individual latitudinal belts, and for 5°×5° squares in central India. Then they calculated a linear trend for each series. The trend was the same in all cases: a warming of 0.4 °C over forty years, which is 0.1 °C per decade. This matches estimates for other regions of the planet, but here the number comes from a single source rather than being pieced together from different bits.
On top of the slow trend lie fast waves. Their period ranges from three to five years, and their amplitude is about ±0.4 °C, almost as large as the entire forty-year trend. These waves are synchronized across all latitudinal belts: when it warms in the south, it also warms in the north. This means the source of the oscillations is not local but common to the whole country. The authors checked what these waves are linked to. Solar activity doesn't fit: the correlation is almost zero. But the flux of galactic cosmic rays gives a correlation of +0.33.
Even more interesting is the connection with El Niño. If you compare India's temperature with the Southern Oscillation Index (a number indicating the strength of El Niño) without any shift, the correlation jumps from zero to +0.34. But if you shift the temperature series forward by one year, the relationship becomes stable. It seems El Niño affects India not immediately, but a year later.
Previously, it was unclear how temperature over India behaves on a decadal scale. Now it's clear: warming is steady, but on top of it run waves that are the same for the whole country. This matters for regional climatologists and agrometeorologists: short anomalies of three to five years can hit crops harder than the slow trend. The thermometer in Delhi has heard the pulse, and now we know its rhythm.
The standard deviation of daily temperatures over India increases with latitude: near the equator it is small, but above 20–25°N it rises sharply due to the contrast between winter and summer. This means that the "average temperature for the whole country" hides very different climatic realities.
In Kim Stanley Robinson's novel 'The Ministry for the Future', a heat wave in India becomes a turning point after which the world begins to seriously combat warming. This article provides the factual basis for such scenarios: not only a slow trend but also rapid oscillations that can amplify extreme episodes.