Most scientists that study the Earth’s climate agree that it’s changing. But why it is changing is the subject of considerable debate.
But several things are beyond dispute.
The amount of carbon dioxide in the atmosphere is increasing. So are the concentrations of other gases such as methane and nitrous oxide. These gases mostly come from human activities: for instance, power plants generating electricity emit large quantities of carbon dioxide; the oil and gas industries emit methane and other chemicals; and the one and half billion gasoline and diesel vehicles on the roads around the world pollute the air with their exhaust gases—mainly carbon dioxide, but also nitrogen gases.
These three gases: carbon dioxide, methane and nitrous oxide (written as CO2, CH4, and N2O) are present in only very low concentrations. But their impact on the global climate is profound.
The greenhouse gases
A greenhouse traps heat because glass is transparent to the short-wave radiation emitted by the sun: the star of our solar system. When this radiation is absorbed on a surface, that surface warms up and emits its own thermal radiation—but at a much longer wavelength. Glass has the unusual property that although it lets through short-wave radiation, it absorbs long-wave radiation—and effectively blocks the transfer of heat by radiation from the inside of the greenhouse to the outside.
Inside the greenhouse, it gets hot.
Carbon dioxide, methane, and nitrous oxide have a similar effect—which is why they are called greenhouse gases. They absorb the long-wave radiation emitted from the surface of the Earth as it is warmed by the sun. As a result, the atmosphere is warmer than it would be if these gases were not present. A warmer atmosphere means a warmer planet. And the oceans too get warmer as they absorb more energy from the atmosphere.
The increase in temperature of the air and the oceans can be measured—but detecting the upward trend in average global temperatures is not easy. Not all scientists agree with the interpretation of the data.
The graph below is from NASA and shows average air and ocean temperatures over the last 60 years. Although there is considerable variation on a yearly basis, the upward trend is clear. This is global warming.
Global average surface air temperatures have increased by about 1°C over the last 115 years. The period we are in now is the warmest in the history of modern civilisation.
So what causes this warming?
The heavy industry, energy production and consumption, a billion and a half vehicles on the roads, agriculture and commerce—all the economic activities of almost 8 billion people on the planet—we all contribute to this warming trend.
Numerous scientific studies point to this conclusion; the most recent being the authoritative report issued in 2017 titled The Climate Science Special Report.
Authored by over 50 top scientists and reviewed by many more, the report is very clear about what is causing global warming. It states in part :
It is extremely likely that human activities, especially the emissions of greenhouse gases, are the dominant cause of the observed warming since the mid-20th century. For the warming over the last century, there is no convincing alternative explanation supported by the extent of the observational evidence.
The majority of the world’s scientists agree with this assessment. You can find the report here.
For more on global warming and climate change see the page here.