The long-lived greenhouse gas released when coal, oil and gas are burned. Concentrations have risen from about 280 ppm before industrialisation to past 420 ppm today.
ClimateWhat is Carbon Dioxide (CO2)?
Carbon dioxide is a colourless, odourless gas made of one carbon atom and two oxygen atoms. It is a natural and necessary part of the atmosphere — plants build themselves out of it, and without any greenhouse gases at all the Earth’s surface would be far below freezing. It is also a trace gas: at present concentrations it accounts for a little over four hundredths of one per cent of the air by volume. Nitrogen and oxygen together make up more than 99%.
That tiny share matters because nitrogen and oxygen are almost transparent to the infrared radiation the Earth’s surface emits, and carbon dioxide is not. CO2 absorbs that outgoing heat and re-radiates it in all directions, including back down. Change the amount of CO2 and you change the planet’s energy balance, which is why a gas measured in parts per million ends up governing the temperature of the monsoon plains.
For a South Asian reader the practical point is this: carbon dioxide is not something you feel or see on a bad-air day. The haze over Delhi in November is particulate matter, not CO2. But the same coal plants, brick kilns, trucks and stubble fires produce both, and only one of the two stays in the atmosphere long enough to change the climate.
How rising CO2 changes the atmosphere
Concentrations are quoted in parts per million (ppm). Before large-scale fossil fuel use, the atmosphere held roughly 280 ppm. Continuous, high-precision monitoring started at the Mauna Loa observatory in Hawaii in 1958, and that record — the Keeling curve — has climbed every decade since, passing 420 ppm in recent years. Superimposed on the rise is a yearly wobble: concentrations fall through the Northern Hemisphere growing season as plants take carbon up, and climb again through the northern winter as leaves decay.
Two properties make CO2 the dominant control on climate rather than just one greenhouse gas among many:
- It is long-lived. A pulse of CO2 is drawn down only slowly. The oceans absorb roughly a quarter of what humans emit, land plants and soils take up a comparable share, and roughly half of each year’s emissions stay in the air — some of it for centuries. Emissions made today are a commitment, not a rental.
- It is not a feedback. Water vapour is a stronger greenhouse gas by mass in the atmosphere, but the amount of water vapour the air can hold is set by temperature. It amplifies whatever change is already underway rather than starting one. CO2 is the knob that gets turned.
The standard measure of how strongly the system responds is the warming expected from a doubling of CO2 from pre-industrial levels. The best current estimate sits near 3°C, with real uncertainty on either side.
Carbon dioxide and South Asia
India is the third-largest emitter of carbon dioxide in absolute terms, behind China and the United States, while its emissions per person remain well below the global average — a gap that sits at the centre of every international climate negotiation the region takes part in. Pakistan and Bangladesh each account for a small fraction of global emissions and a very small amount per person. All three countries are among the most exposed to the consequences.
The regional sources are distinctive. Coal-fired electricity dominates India’s power mix. Brick kilns run in their tens of thousands across the Indo-Gangetic plain, Punjab in Pakistan and central Bangladesh, mostly in the dry winter months. Cement and steel production, diesel freight and the rapid growth in road vehicles and air conditioning add the rest.
India has stated a target of net-zero emissions by 2070, announced at the COP26 conference in Glasgow in 2021, alongside large-scale solar and wind deployment.
What this means for weather in the region
Rising CO2 does not create new kinds of weather in South Asia. It loads the dice on the kinds already here.
- Heat. Pre-monsoon heat in the northern plains is already dangerous in a normal year. A warmer baseline means the same synoptic pattern produces a hotter, longer and more frequently repeated event, and pushes more days into the range where humidity makes cooling by sweating ineffective.
- Rainfall intensity. Warmer air holds more moisture. The monsoon’s seasonal total is governed by circulation and is hard to predict, but the tendency for a season’s rain to arrive in fewer, heavier bursts — with dry gaps between them — is a direct consequence of a warmer atmosphere, and it is the pattern that floods cities.
- Sea level. Warming water expands and land ice melts. For the Bengal delta and the Indus delta, a slowly rising baseline turns an ordinary cyclone storm surge into a damaging one and pushes saltwater further into farmland and drinking water.
CO2 versus the air quality you can measure
The two are constantly conflated. An air quality index is built from particulate matter, ozone, nitrogen dioxide, sulphur dioxide and carbon monoxide — pollutants that harm lungs at the concentrations found in cities. Carbon dioxide outdoors is not a health hazard in that sense; nobody gets asthma from 420 ppm.
The link between them is the source. That is why cutting coal use, cleaning up brick kilns and moving freight off diesel pays twice: cleaner air in Lahore or Dhaka within days, and slower warming over decades.
Frequently asked questions
How much CO2 is in the air right now? A little over 420 parts per million, and rising by a couple of parts per million each year. The reference measurement comes from a global network of background monitoring stations, of which Mauna Loa is the longest-running.
Why is 420 ppm a problem if it is such a small number? Because the gases that make up almost all of the atmosphere are transparent to infrared radiation and CO2 is not. Effectiveness, not abundance, is what matters. The pre-industrial value was 280 ppm, so the increase is already more than half again.
Does CO2 affect the air quality reading for my city? No. Air quality indices in India, Pakistan and Bangladesh are calculated from particulate matter and reactive gases. CO2 is not included and is not measured by city monitoring stations.
Will cutting emissions cool things down quickly? Surface temperature stops rising fairly soon after emissions reach net zero, but it does not fall back — the CO2 already added stays for a long time, and sea level keeps rising for centuries. The realistic goal is to stop making the problem worse.
Where can I see how heat and rainfall are actually behaving in my city? Every city page carries live conditions, the multi-day forecast and air quality together, which is the practical end of all of this. Start with Delhi, Mumbai, Kolkata, Dhaka, Lahore or Chennai.