When you arrive in Mumbai's international airport, you are greeted by a huge advertisement put up by Harbin Electric Corporation, one of the largest makers of turbines and boilers, used in India's power plants. The electric billboard also shows smokestacks spewing white smoke. It's all a sign of great industrial progress in Harbin, China's 10th largest city.

But Harbin is lately choking in its smog. The smog is so thick that schools, flights and the highway had to be closed recently. The smog pollution level was 40 times worse than what is permissible by WHO standards, and is causing breathing ailments. The winter sun is not visible for days in this northern city. But Harbin is not an exception. During the winter, even Beijing and Shanghai suffer from smog. Pollution has become an allyear phenomenon, not just in winter.

Last month American jazz singer Patti Smith had to cancel her Beijing concert due to an asthma attack triggered by pollution. China's many cities have tall skyscrapers, which are heated by coalfired mini-plants during winters. The smoke makes the pollution much worse. Coal prices dropped drastically due to gas discovery in America. Hence coal-fired thermal power plants are picking up. Electricity shortage means burning more coal. It's not just air quality. China's water quality has also deteriorated, and rivers and forests are being degraded as well.

The environment versus economic growth tussle is most visible in China, and its impact is felt in every home, struggling with asthma or shortage of potable water. But India is not far behind in experiencing this tussle. This trade-off between environmental quality and economic prosperity is no longer just a luxury to be indulged by rich countries. It affects us as well.

This great tussle is also not just about air and water quality. It is about climate change and global warming too. As per the latest report of the Intergovernmental Panel on Climate Change (IPCC), global warming is causing more frequent occurrences of hurricanes, floods and famines, and will lead to severe food shortages. This means the cloudburst over Kedarnath or hurricane Phailin were not freak or rare phenomena, and might recur more often.

The one metric that captures the state of global warming is the amount of carbon dioxide. The more carbon we burn, the more the CO2, the greater the risk of all the disasters that IPCC predicts. It does not matter who contributes to decarbonising: coal burnt in India, or diesel burnt in Germany or factories running in China. All deplete carbon from the global stock, and decarbonising affects the entire globe. Based on IPCC science, PwC the accounting firm estimated that the world has a budget of 270 billion tonnes of carbon that can be emitted until the year 2100, to prevent global warming beyond 2 degrees Celsius. That's our budget. This budget cannot be exceeded.

There is no budget deficit that can be financed by borrowing from some bank. Unfortunately the world is running out of this budget too fast. We are emitting almost 34 bn tonnes annually (50% by just three countries, China, USA and India), and emissions are growing at 6% annually. So this budget will exhaust in less than 15 years, unless we cut emissions drastically. Remember there is no concept of deficit. If you exceed the carbon budget, you trigger many Phailin-type events. The punishment for exceeding the budget is often felt by "innocent bystanders". But since no single country manages this budget, you can't point any fingers at anybody, except to ourselves.

Thus what happens in Harbin is a harbinger for Mumbai or Delhi as well. Managing our carbon budget is the toughest challenge before humanity today. But don't wait for Harbin. Let's act today. Now.