[Explainer] Why is it getting hotter?

“The earth has a fever,” scientists say rather poetically. This means that it is beginning to cross the thresholds of 1.5 and 2 degree Celsius (°C) temperature rise compared to the 1850-1900 “pre-industrial” average. That exposes people to climate change impacts such as heat stress. Scientists say that human-induced global warming, boosted temporarily by El Niño (a natural exaggeration of the seasonal warm season of the tropical eastern Pacific that occurs every two to seven years), is to blame.

The signs of the warming trend were bright and clear in 2023 – heat waves across the Mediterranean region and the United States, wildfires in Canada, Greece, Australia and Indonesia, floods following a lingering drought in the Horn of Africa.

After 2023, the warmest year so far since the temperature data recording began globally in 1850, the warmest January was recorded.  “Since the 1980s, each decade has been warmer than the previous one. The past nine years have been the warmest on record,” the World Meteorological Organization notes. The  UK Met Office forecasts 2024 could be still warmer than 2023.

There have been hot weather advisories for Kerala and elsewhere in India. In Thiruvananthapuram, traditional fishermen said they are taking extra crates of drinking water to the sea, to stay hydrated in the warm weather.

Which temperature records were broken?

The global average temperature for 2023 was 14.98°C, which was 1.48 °C warmer than the 1850-1900 average and 0.17 °C more than the next warmest year of 2016. For the first time, a rise of 1.5°C — the limit set by the Paris climate agreement — or above, cannot be ruled out this year, the U.K. Met Office reports.

January 2024 recorded the highest ever monthly global surface temperature, 1.27°C above last century average of 12.2°C. It is the highest for the 8^th consecutive month as El Niño conditions prevail in the eastern equatorial Pacific Ocean. These conditions will last till April-June 2024, NOAA’s Climate Prediction Center notes. That makes it the warmest January on record, eighth consecutive month of record warmth. It was also the second-wettest January.

Records were broken in 2023. Never before every day of the year had recorded 1°C above the 1850–1900 levels, as reported by the  Copernicus Climate Change Service (C3S). About half the days were 1.5°C warmer. In the second half of the year, each month was warmer than that month of any year earlier. Further, 2023 had record temperatures in absolute terms in July, and relative to the annual cycle in November, and from June 9 on almost all days the warmest on the ERA 5 data record. Two days in November 2023 were more than 2°C warmer.

Above-average temperatures were reported from land and sea areas across the world in 2023. For large areas of Europe and North America, and elsewhere, annual temperatures were more than 1°C higher than the 1991-2020 average.

The poles also bore the brunt of climate change. Antarctic sea ice extent shrank to the fifth lowest for January, according to data from NOAA’s National Centers of Environmental Information (NCEI). Arctic temperatures remained above average. Global sea ice extent was 6.90 million square miles, that is 440,000 square miles less than the 1991–2020 average. While Arctic sea ice extent was slightly below average, Antarctic sea ice extent was much below average by 420,000 square miles, NCEI reported. The pace of warming, however, is generally higher over land areas than oceans.

What are the contributing factors?

The warming trend has human-made and natural influences. The primary factor that drives it is climate change. Then 2023 was an El Niño year, a case of natural climate variability or the way climate components such as temperature and precipitation differ from an average. NASA Earth Observatory has listed three more contributing factors  – ocean warming, a decrease in aerosols, and the 2022 Hunga Tonga-Hunga Ha‘apai undersea volcanic eruption in Tonga, South Pacific.

Average air temperatures close to the earth’s surface have been increasing. A key contributor of this change is the excess carbon dioxide emitted from burning of fossil fuels such as coal, gas, and oil that light up our homes, businesses and cities and keep us mobile. Farming and dairy keeping, land-use change, construction, waste management, and industrial processes emit methane, nitrous oxide, and various synthetic chemicals. These gases blanket and trap heat around the earth.

Read more: [Explainer] Why mitigating methane emissions matters

Warming due to climate variability adds to climate change impacts. El Niño modifies the atmospheric flow enough to change local weather in many parts of the world for 6 to 12 months. Coupled with a phenomenon called Southern Oscillation that denotes changes in sea level air pressure patterns in the Southern Pacific Ocean between the Tahiti island and Darwin, Australia. During El Niño conditions, the average air pressure is higher in Darwin than in Tahiti and vice versa during alternating cool phase called La Niña. Together called El Niño Souther Oscillation (ENSO), this phenomenon of natural climate variability influences local weather patterns in different remote parts of the world. In El Niño years, Indian summer monsoon get suppressed. Temporary warming of the ocean pushes the Pacific jet stream south, making parts of northern U.S. and Canada warm and dry.

Over tropical oceans, temperature anomalies shifted from negative in 2022 to positive in 2023, in line with La Niña transitioning to El Niño over the Pacific Ocean and influenced by the warm Atlantic and Indian Oceans, the warmest among all oceans.

The air temperature anomalies are closely linked to sea surface temperature anomalies. An anomaly denotes a departure from a reference value or long-term average. In this case, positive or negative anomaly indicates warmer or cooler temperature than the reference value. Upper ocean heat content — the amount of heat stored in the top 2,000 metres of the ocean — and sea surface temperature set new records in 2023. Ocean heat content is a key climate indicator because the oceans store 90% of the excess heat in the Earth system and in the world’s oceans, it increased by the greatest margin ever in 2023.

Small particles in the air such as smoke, soot, volcanic ash and sea spray called aerosols can absorb or reflect sunlight and causing slight warming or cooling of the air, respectively. Dark aerosols generally absorb heat and light ones reflect it.  They often have a cooling effect, though minimally and with limited certainty and complex influence on weather patterns. As new government norms around the world cut down air pollution and clear the air, aerosols concentrations have been decreasing worldwide, possibly making a tiny warming effect that scientists are trying to understand better.

The Hunga Tonga-Hunga Ha‘apai volcanic eruption in January 2022 injected huge quantities of water vapour and sulphur dioxide into the atmosphere. Water vapour is a greenhouse gas with a warming effect. However, sulphur dioxide rapidly converted to sulphate aerosols that reflect back solar radiation with a cooling effect. So, in effect, they cancelled each other out and did not add to the record heat in 2023, NASA scientists note.

What are the possible impacts of extreme heat?

A recent UN Intergovernmental Panel on Climate Change (IPCC)  special report forecasts serious impacts beyond the 1.5°C mark – more and intense rainfall, droughts and heatwaves. Crossing the “critical threshold” of 2°C could expose us to compound impacts such as changes in climate variables such as air temperature, precipitation (rain, snow etc.), relative humidity, solar radiation, and wind speed, a recent study warned. Heat stress and fire risk will be more in a changed climate, they add.

Severe heat waves and droughts are two of the most pronounced impacts, affecting about 14 percent of the people at least once every five years at 1.5°C warming, and 37 percent at 2 degrees, as a report based on the IPCC data notes.

A heat wave is a marked warming or invasion of very warm air over a large area. It involves unusually hot weather for at least two consecutive days during the hot period of the year, crossing the local thresholds, as the World Meteorological Organization (WMO) defines. More than one-fifth of people are already experiencing a temperature rise above 1.5°C in at least one season. “At 2°C warming, the deadly heatwaves India and Pakistan saw in 2015 may occur annually,” scientists pointed out.

The India-Pakistan heat wave of April-June 2015 resulted in over 3,600 deaths. India Meteorological Department declares a declares a heat wave when the surface air temperature rises by 4.5–6.4°C above the normal daily maximum temperature of 40°C or more for plains and at 30°C or more or more for hilly regions. When the rise is more it is called a severe heat wave. They expose people to health issues, food shortages and increased risks of death, as a 2003 study based in India showed.

In a 2°C warmer world, about 61 million more people in cities would be exposed to severe drought compared with 1.5 degrees warmer globe. However, limiting warming to 1.5°C can reduce drought and water stress, as IPCC notes.

Other significant impacts of a warming world include warmer winters and less cold days and nights. For instance, closer to the poles, the coldest nights will be 4.5°C warmer at 1.5 degrees of warming.

UK Met Office forecast for 2024 is in line with the current warming trend of 0.2°C per decade. As Nick Dunstone, who led the study, said, there is “a reasonable chance” of a year temporarily exceeding 1.5°C rise in temperature.

Read more: What are El Niño and La Niña? How do they affect the Indian monsoon?

Banner image: The earth’s temperature is beginning to cross the thresholds of 1.5 and 2 degree Celsius (°C) temperature rise compared to the 1850-1900 “pre-industrial” average, exposing people to climate change impacts such as heat stress. Photo by Yogendra Singh/Pexels.