The warming produced when gases in the atmosphere absorb heat radiating from the surface and send part of it back down. It is natural and necessary; human emissions have strengthened it.
ClimateWhat is the Greenhouse Effect?
The greenhouse effect is the reason a clear, humid night in Kochi or Chittagong stays warm until dawn while a clear night in the Thar desert can drop by twenty degrees or more. The difference is not the sun, which has set in both places. It is what is in the air above: moist coastal air absorbs the infrared radiation streaming up from the ground and sends much of it back, while the dry desert air lets it escape to space.
Scaled up to the whole planet, that is the greenhouse effect. Without any of the gases that do this absorbing, the Earth’s surface would average roughly 33°C colder than it does — far below freezing, with no liquid water and no monsoon. The effect is not a problem in itself. It is the precondition for the climate South Asia farms in.
The problem is that the effect has a dial, and burning fossil fuels turns it up. Carbon dioxide is more than half again its pre-industrial concentration, and the extra absorption is the mechanism behind the warming recorded across India, Pakistan and Bangladesh.
How the greenhouse effect actually works
The name is a bad one. A glass greenhouse stays warm mainly because the glass stops warm air from rising away — it suppresses convection. The atmosphere does something different, and the difference explains why the effect keeps working at night and in winter.
Radiation comes in two kinds here, sorted by temperature:
- Incoming sunlight comes from a surface at about 5,500°C and arrives mostly as visible and near-infrared light. The atmosphere is largely transparent to it. It passes through and heats the ground.
- Outgoing radiation from the Earth comes from a surface near 15°C and is emitted in the thermal infrared, at much longer wavelengths. Here the atmosphere is not transparent. Water vapour, carbon dioxide, methane and other trace gases have absorption bands right across this range.
Absorbed infrared energy is re-emitted in all directions, including downward. The surface therefore receives energy twice — once from the sun, once from the air above it — and has to settle at a higher temperature to shed it all.
Adding more greenhouse gas raises the effective altitude from which the planet’s heat finally escapes to space. Higher up is colder, a colder body radiates less efficiently, and the whole column has to warm until the books balance again. That is the physics behind everything else on this page.
Which gases matter
Not all greenhouse gases play the same role, and the distinction matters for what policy can change.
- Water vapour is the largest contributor to the natural effect. But the amount of water the air can hold is fixed by temperature, so vapour responds to warming rather than causing it. It is a powerful amplifier: warm the planet slightly for another reason and the extra vapour roughly doubles the effect. Nobody controls it directly.
- Carbon dioxide is the control knob. It is long-lived, it does not condense out, and its concentration is set by emissions.
- Methane is far more powerful per tonne over a century, but survives only about a decade in the atmosphere. In South Asia its main sources are flooded rice paddies across the Gangetic plain and the Bengal delta, livestock — India has one of the world’s largest cattle populations — landfills and leaks from gas infrastructure. Because it is short-lived, cutting methane produces results within a human lifetime.
- Nitrous oxide comes largely from nitrogen fertiliser, applied heavily across the irrigated plains of Punjab, Haryana, Sindh and Bangladesh. Ozone in the lower atmosphere is both a pollutant and a greenhouse gas, and forms readily in the hot, sunlit, traffic-heavy air of large South Asian cities.
The greenhouse effect in South Asia
The regional fingerprint shows at night too. A strengthened greenhouse effect slows the rate at which the surface loses heat after sunset, so minimum temperatures rise along with maximums. In practical terms that is the difference between a heatwave people recover from overnight and one they do not: the body needs the night to shed accumulated heat, and warm-night heatwaves in Delhi, Lucknow, Karachi or Dhaka are the ones that fill hospitals.
Other regional consequences follow from the same physics:
- Humid heat. More vapour in a warmer atmosphere means more days when high temperature and high humidity combine, along the coasts and in the eastern plains, to a point where sweating no longer cools the body effectively.
- Rainfall bursts. A warmer atmosphere carries more moisture, and when the monsoon does deliver, it can deliver more in a shorter time. Seasonal totals are governed by circulation; the intensity of individual spells is much more directly tied to warming.
- Himalayan ice. Snow and glacier melt sustain dry-season flow in the Indus, Ganga and Brahmaputra. Warming works through that store slowly and irreversibly, and hundreds of millions of people live downstream of it.
Why the regional picture is more tangled than the global one
South Asia is not a clean laboratory for greenhouse warming, because the same activities release enormous quantities of aerosols — sulphate particles, dust and black carbon — that form the persistent dry-season haze over the Indo-Gangetic plain.
These particles cut the sunlight reaching the ground and, in the case of black carbon, warm the air aloft while shading the surface. The net regional effect has been to mask part of the greenhouse warming and, according to a substantial body of research, to weaken the land–sea temperature contrast that drives the summer monsoon. Cleaning up the air, which is unambiguously good for health, removes some of that masking. The underlying warming does not go away — it becomes more visible.
Frequently asked questions
Is the greenhouse effect bad? No. The natural effect keeps the planet roughly 33°C warmer than it would otherwise be and makes South Asian agriculture possible. What is harmful is the additional warming from gases humans have added on top.
If water vapour is the strongest greenhouse gas, why blame CO2? Because water vapour is controlled by temperature, not by emissions. It leaves the atmosphere within days as rain. CO2 stays for centuries and sets the temperature that determines how much vapour the air holds.
Why do nights matter so much in a heatwave? The greenhouse effect operates continuously, including after sunset, and a stronger one slows overnight cooling. When night minimums stay high, the body never gets the recovery period it needs, which is why humid-night heat events are more dangerous than their daytime peak suggests.
Where can I see how this is playing out in my city? Check current conditions, night-time minimums and the forecast on any city page: Delhi, Karachi, Dhaka, Kolkata, Lucknow or Bikaner, where the desert diurnal range is at its most dramatic.