Climate Change 2021: The Physical Science Basis

The report states that human beings have contributed significantly to the warming of the planet, including the atmosphere, land and sea. In the last 40 years, each decade has been successively warmer than the previous one.

The average global surface temperature was 0.99°C higher in 2001-2020 than in 1850-1900. If current carbon dioxide emissions are kept low, the average temperature is likely to see a 1-1.8°C rise by 2081–2100. If emissions continue being produced at a high rate, the average temperature is likely to increase by 3.3-5.7°C by then.

The impact of past and future greenhouse gas emissions – especially on the ocean, ice sheets and global sea level – is likely to be irreversible for centuries. Changes in global ocean temperatures and deoxygenation will be beyond repair in the next 100-1,000 year time period.

Extreme weather events such as heatwaves, heavy precipitation, droughts, and tropical cyclones, have intensified across the globe due to human-induced climate change since the IPCC’s fifth assessment cycle of 2013-14. Marine heatwaves have intensified and doubled in frequency since the 1980s. Most heatwaves after 2006 have been attributed to human-induced warming.

All scientific projections predict that global warming will increase until at least the middle of the 21st century. The global temperature is likely to rise by 1.5 to 2°C during this time unless there are drastic cuts in greenhouse gases emissions.

The rise in global temperatures are accompanied by an increase in the severity of extreme climate and weather conditions like marine heatwaves, heavy precipitation, agricultural and ecological droughts, and tropical cyclones. It is very likely – the report states – that heavy precipitation events will become more frequent and of greater intensity with the rise in temperature. Extreme daily precipitation events are projected to intensify by about seven per cent for each 1°C rise in global temperatures.

About two-thirds of the global coastline is likely to see a considerable rise in regional sea levels relative to the average global increase. This will heighten the frequency and intensity of events like coastal flooding. The frequency of ‘extreme sea level events’ such as coastal erosion and flooding – which usually occur once in about 100 years in many regions – may increase to once a year.

The report states that urbanisation leads to human-induced warming starting with immediate consequences the local level. When coupled with more frequent hot weather extremes, urbanisation processes are likely to cause increasingly severe heatwaves, heavy precipitation and high runoff intensity.

As per the report, controlling climate change within a sustainable range can only be achieved through net zero carbon dioxide emissions, and a rapid reduction in the emissions of other greenhouse gases like methane. With every 1,000 gigatonnes of carbon dioxide emissions, the global surface temperature is likely to increase by 0.27°C to 0.63°C.

In 2020, the worldwide spread of Covid-19 led to a drop in carbon dioxide emissions which caused a discernible yet temporary change in air pollution levels. This led to a small, temporary increase in ‘total radiative forcing’ – defined as the net change in the energy balance of the Earth system due to an imposed disturbance – primarily due to cooling caused by a reduction in aerosols emerging from human activities. The reduction in greenhouse gas emissions also led to better air quality. Despite these improvements, many polluted regions failed to achieve the air quality guidelines prescribed by the World Health Organization during this period.

Focus and Factoids by Udita Mukherjee.

PARI Library's health archive project is part of an initiative supported by the Azim Premji University to develop a free-access repository of health-related reports relevant to rural India.