Not a single cool month in the last 628 months

Over 80 per cent of the world’s population—born during the last 50 years—is living in an earth afflicted with a fever that simply refuses to subside. One could call this the generation that grew with a new atmosphere; badly maligned by humans. In April 2017, scientists from Climate Central—an international association of scientists and journalists reporting and researching climate change—released a stunning chart depicting a month by month temperature rise since 1880. “There has not been a cool month in 628 months.” Climate Central crunched enormous amount of data from the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA) into a stark one-page chart that showed months with higher temperatures than the baseline of average global temperatures between 1881 (coinciding tentatively with the Industrial Revolution) and 1910 in red colour.

The blue colour represented relatively cooler months. Finally, the chart appeared smudged completely with red colour. For the last 50 years, every month reported warmer than the early industrial baseline. Brian Kahn of Climate Central said, “Cool blues have been disappearing and replaced by a wave of unending heat. Climate change is likely to continue the streak of warmer than normal months into the foreseeable future as temperatures keep marching upward.”

Seven months before this, researchers from the University of Reading in the UK compiled 167 maps of yearly global temperature side by side for 1850-2016. These maps showed global historical surface temperature anomalies relative to a 1961-1990 baseline. It drew a similar conclusion: continuous rise in annual surface temperature. But it brought out another trend: surface temperatures had risen dramatically, especially since the 1990s.

For Indians, and to some extent people from all continents, this didn’t come as a surprise. Since this period, the world had reported the warmest years in quick succession. Since 1992, heat waves killed 22,562 people in India. In the last 23 years, India had no fewer than 393 deaths each year due to heat waves. Between 1992 and 2004, the annual death toll crossed 1,000 twice—in 1995 and 1998. Since then, more than 1,000 people died in seven heat waves. The worst summer in terms of the sheer number of casualties was 2015 when 2,422 people died. Till then, 1998 was the warmest year in a century.

The July 2016 temperature across global land and ocean surfaces was about 1.57°F above the 20th century average of 60.4°F. At a time when every month seemed to be breaking some climatic records, this was the highest for July in the 1880–2016 period, surpassing the previous record set in 2015 by 0.11°F , the previous record holder for the warmest month on record. July 2016 marked the 40th consecutive July with temperatures at least nominally above the 20th century average. July 1976 was the last time global land and ocean temperatures were below average. July 2016 had the lowest monthly global temperature departure from average since August 2015 and tied with August 2015 as the 15th highest monthly temperature departure among all months (1,639) on record.

Science had definitely diagnosed the fever; it was also sure by now that the earth was under a warming spell for more than a century. For over four decades the debate was on whether the warming was “natural” or “human induced”. The scientific basis of this doubt on attribution is the fact that the earth passed through alternative phases of cooling and warming. The Ice Ages were the most striking arguments tossed around to convince that the current warming spell was just natural. However, there was not much scientific evidence against the verdict that global warming was due to human factors. And the climate anomalies were a result of these factors.

Earth’s climate is a complex, interactive system consisting of the atmosphere, land surface, snow and living things; like you and me and the pretty trees and tigers. The atmospheric component of the climate system most obviously characterises climate; hence, climate is generally defined as “average weather”. As such climate change and weather are intertwined. Observations can show there have been changes in weather, and it is the statistics of changes in weather over time that identifies climate change. The climate system evolves in time under the influence of its own internal dynamics. It can also evolve due to changes in external factors that affect climate—these factors are called “forcing”. External “forcings” include natural phenomena, such as volcanic eruptions and solar variations, as well as human-induced changes in the chemical composition of the atmosphere.

First, let’s understand the very fundamentals of the earth’s climate system because this precisely drove global politics over climate change: who caused it and who to be held responsible? Radiative energy from the sun powers the climate system. About 30 per cent of the sunlight that reaches the top of the atmosphere is reflected back into space. Roughly two-thirds of this reflection is due to clouds and small particles in the atmosphere known as “aerosols”. Light-coloured areas of earth’s surface—mainly snow, ice and deserts—reflect the remaining sunlight.

The energy not reflected back to space is absorbed by the earth’s surface and atmosphere, and amounts to approximately 240 watts per square metre (W/sq m). To balance the incoming energy, the earth itself must radiate, on an average, the same amount of energy back to space. The earth does this by emitting outgoing long wave radiation. Everything on earth emits long wave radiation continuously. The warmer an object, the more heat energy it radiates.

To emit 240 W/sq m, a surface would have to have a temperature of -19°C. This is much colder than actual conditions at the earth’s surface. The requisite -19°C is found about 5 km above the surface. So, how is the earth warmer?

This question led to the discovery of the greenhouse effect, and also to unravel the human causes behind global warming. Ed Hawkins, who worked for the UK National Centre for Atmospheric Science, wrote: “In the 1820s, the French mathematician Joseph Fourier was trying to understand the various factors that affect Earth’s temperature. But he found a problem—according to his calculations, the Earth should have been a ball of ice. The most obvious factor, the Sun, did not seem to provide enough energy to raise the temperature of Earth above freezing. Fourier’s initial ideas, that there must be additional energy coming from the Earth’s core or from the temperature of outer space, were soon dismissed. Fourier then realised that the atmosphere, which at first seemed transparent, could be playing a crucial role.”

The global mean surface temperature of earth is about 14°C. This is due to the presence of gases which act as a partial blanket for the long wave radiation coming from the surface.

This blanketing is known as the natural greenhouse effect.

The glass walls in a garden greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the earth’s greenhouse effect warms the surface of the planet. Without the natural greenhouse effect, the average temperature at earth’s surface would be below the freezing point of water. Thus, earth’s natural greenhouse effect makes life as we know it possible. And global warming had other unnatural reasons. Let’s dive deep into the history of how we came to this conclusion.

(This story was published in Climate Change Now book under the title ‘Science’s tumultuous postulates’)