During the billions of years of our planet’s existence, its climate has varied a lot. At times the entire planet may have been wholly or mostly enveloped by snow and ice (‘Snowball Earth’), whilst at other times Polar Regions were inhabited by tropical animals. Even in the roughly hundred-thousand years of Homo sapiens’ tenancy, ice ages have come and gone. The most recent 8,000 years or so, since the beginning of agriculture and appearance of urban areas, however, have been unusually steady.

Over this time, ice-core records show clearly that levels of carbon dioxide (CO2) in the atmosphere were around 280 parts per million (ppm), give or take 10 ppm. Predominantly CO2 and other Greenhouse Gases (GHGs) in the atmosphere then acted (and are still acting) like a blanket or a cap for trapping some of the heat that Earth might have otherwise radiated into space.

But how exactly do molecules of GHGs trap heat? The answer requires diving into physics and chemistry. The atmosphere surrounding our planet is so thin that it is highly vulnerable to the drastic changes in the chemical composition brought about when we recklessly and constantly pollute it with prodigious volumes of GHGs as chemical wastes.

The growing blanket is overwhelming the atmosphere’s ability to mediate balance between the Earth and the sun, trapping more extra heat energy each day in the lower atmosphere than would be released by 400,000 Hiroshima atomic bombs.

As mentioned, the Earth warms as a result of solar radiation that reaches the surface and is not immediately reflected back to space. By how much does the Earth warm? By just enough that it reaches a temperature at which the Earth radiates to the space at the same rate that that sun transmits energy to Earth. The key to understanding the energy balance is a concept known as ‘Black-body radiation’, a term used to describe the relationship between an object’s temperature, and the wavelength of electromagnetic radiation it emits.

Any warm body, including the Earth itself, radiates electromagnetic energy as infrared (long-wave) radiation. The warmer the body, the greater the radiation. When the sun radiates ultraviolet (shortwave) radiation to Earth, the Earth warms to just the temperature at which the Earth radiates energy to the sun, equal to the sun’s radiation reaching the Earth.

The energy balance is thereby reached. However, while sunlight reaches the earth, the surface absorbs some of the light’s energy and radiates it as infrared waves, which we may feel as heat. Holding our hands over a dark rock on a warm sunny day we can feel the phenomenon.

The infrared waves travel into the atmosphere and will escape back into space if unhindered. Oxygen and nitrogen in the atmosphere do not interfere with infrared waves, because their molecules absorb energy that has tightly packed wavelengths of around 200 nanometers or less. Infrared energy travels at wider and lazier wavelengths of 700 to 1,000,000 nanometers.

So they let the waves (and heat) pass freely through the atmosphere. But with CO2 and other GHGs, it’s different. For example, CO2 absorbs energy at a variety of wavelengths ranging between 2,000 and 15,000 nanometers ~ a range that overlaps with that of infrared energy. As CO2 soaks up this infrared energy, it vibrates and remits the infrared energy back in all directions. About half of that energy goes out into space, while about half returns to Earth as heat, contributing to the greenhouse effect.

The basic greenhouse effect is a lifesaver for us. If the Earth, like the moon, had no GHGs, then it would be a much cooler place and would not support life. Without greenhouse effect, the average Earth temperature would be around minus 14 degree centigrade, well below the freezing point of water.

Besides CO2, there are several major GHGs: methane (CH4), nitrous oxides (N2O), some industrial chemicals called hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF4) and water vapour (H2O). There are a number of salient points about GHGs. The most important is the ‘residence time (or lifetime)’ which is defined as the average time a GHG takes for its molecule to be removed from the atmosphere.

These gases can remain in the atmosphere for different amounts of time, from months to millennia, and affect the climate on different time scales. The other most important factor of a GHG is its ‘radiative forcing’ which, as defined by the IPCC, is a measure of the influence a given climatic factor has on the amount of downward-directed radiant impinging upon Earth’s surface.

Primarily human activities and also those by natural forces cause climatic factors. ‘Positive forcing’ exerted by climatic factors contribute to the warming of Earth’s surface, whereas ‘negative forcing’ cools Earth’s surface. The lifetime of CO2, the most significant man-made GHG, in the atmosphere is very difficult to determine; because there are several processes that remove CO2 from the atmosphere. Between 65 to 80 per cent of CO2 released into the air dissolves into the ocean over a period of 20–200 years.

The rest is removed by slower processes that take up to several hundred thousand years. This means that once in the atmosphere, CO2 can continue to affect climate for thousands of years. Methane traps roughly 23 times more heat than CO2, but its residence time is much shorter, around ten years. Nitrous oxide is destroyed in the stratosphere and removed from the atmosphere more slowly than methane, persisting for around 114 years.

GHGs containing chlorine and /or fluorine (CFCs, HCFCs, HFCs, PFCs) include huge numbers of different chemical species, each of which can last in the atmosphere for specific lengths of time ~ from less than a year to many thousand years. The warming effect of all the anthropogenic CHGs is determined by adding up the separate radiative forcing of each of the six GHGs (CO2, CH4, N2O, HFCs, PFCs and SF6).

For each GHG, the radiative forcing is measured in unit of CO2 equivalent (CO2E). For example, since CH4 has a radiative forcing equal to 23 times that of CO2, each CH4 molecule in the atmosphere is counted as equivalent in warming potential to 23 molecules of CO2. In 1972, the big concern worldwide was that humanity would run out of key minerals or ores and the resulting scarcity would make it difficult to maintain the level of economic activity, much less to continue to achieve economic growth.

What was not then clearly appreciated was that the real limits were not the minerals, but the functioning of the Earth’s ecosystems, the biodiversity and the ability of the atmosphere to absorb anthropogenic GHGs. Interestingly, even though the core idea of planetary boundaries (PB) was first being understood around five decades ago, the kinds of boundaries that would turn out to be most important were not then very clear to the scientific community.

Subsequently, we began to appreciate that the real PB are mainly ecological rather than limits of mineral ores. Indeed, there is no doubt that the greatest of these threats is humaninduced climate change, coming from the build-up of GHGs including CO2, CH4, N2O and some other industrial chemicals. The most important aspects of climate change are: (1) It is a global crisis. It affects every part of the planet. Ban Ki-moon, former SecretaryGeneral of UN, aptly said: ‘Climate change carries no passport and knows no national borders. Countries must work towards the common interest, beyond narrow national interests.’ (2) The problem crosses not only countries but generations.

People who are going to be most profoundly affected by humaninduced climate change have not yet been born. (3) The challenge is complicated because the problems of GHG emission go the core of modern economy. The success of modern economic growth arose from the tap into fossil fuels energy.

The number one human contributor to climate change is the burning of fossil fuels that emit CO2 into the atmosphere and thereby change the planet. We must transplant fossil fuels energy with an alternative based on low-carbon energy. (4) Climate change is a slow-moving crisis. While rapid in epochal terms, it is very slow from the point of view of daily events and the political calendar. If the climate crisis were to culminate in a single event in a year’s time, there could be little doubt that humanity would get itself organized to prevent or adapt to the crisis.

The changes year-to-year may be too gradual to provoke large-scale political actions, yet the cumulative effects could prove devastating. What does the future hold? The Centre for Science and Environment (CSE), a Delhi–based NGO, analyzed climate changerelated datasets, and mapped future scenarios going forward in its fortnightly magazine, Down To Earth.

The most important findings are: * For over 6000 years, human have restricted their habitat settlements to an annual average temperature of between minus eleven to fifteen degree Celsius (-110 to 15OC). As of 2020, only 0.8 per cent of the world’s land surface experiences annual temperature of more than 29OC or over. But in a warming world, if emission of GHGs continues, this range could rise to 19 per cent of the Earth’s surface, affecting three of the projected nine billion people by 2070. India would be one of those worst-hit countries in Asia. *

In a warming world, ‘climate suitability’ would stretch to even the sparsely populated Artic regions of the world. This expansion of climate niche threshold could trigger a wave of migration. * According to a World Bank report, by 2050, at least 216 million people will be migrating within their own countries. Migration being a dynamic event, influenced by socio-economic and political factors, the reason might vary from region to region.

According to the report, 40 per cent of total migration will happen in sub-Saharan African countries where water scarcity will be the main driving force. Indeed, a global crisis has unfolded quickly, and, as in a classic Greek tragedy, we have been told what the future may hold. But so far we seem unable to step away from the path to disaster that has been mapped out for us. The last act is about to begin.

(The writer is a retired IAS office)