IPCC Sixth Assessment – The Current State

Last month the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report Summary for Policymakers was released. This explains the science behind climate change in a fairly concise and authoritative way. For people with a couple of hours to spare it is well worth the read. For others we have taken some of the key points in this series of easy-to-read posts.

How is the climate changing and how much is as a result of human influence? How might the climate continue to change in the future with the different emissions scenarios?

The first section of the report details the current state of the climate based on detailed scientific observation.

“It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred.”

IPCC Sixth Assessment Report, August 2021

Since 1750, changes to the environment are considered man-made – coinciding with the industrial revolution.

Concentrations of greenhouse gases (GHGs) have continued to increase in the atmosphere, reaching annual averages of 410 ppm (410 parts in a million) for carbon dioxide (CO2), 1.9 ppm (1.9 parts in a million) for methane (CH4), and 0.3 ppm (0.3 parts in a million) for nitrous oxide (N2O) in 2019.

To put this into perspective, over the last million years CO2 has not risen above 300 ppm until the early 1900s, and CO2 continues to rise. Since 1950 both methane and nitrous oxide have risen sharply.

Each of the last four decades have been successively warmer than the last.

Temperature increase between 1850-1900 and 2010-2019 has a strong degree of confidence at 1.07°C. Scientists believe 1.5°C is the ideal limit.

Man-made changes have led to more rain – and storm tracts have moved poleward in both hemispheres, as have climate zones. Retreat of glaciers in the northern hemisphere is very likely dominated by man-made changes to the planet.

The upper sea level (to 700 m down) is warmer, creating thermal ocean expansion, and there is greater acidity from polluting factories. The sea has risen circa 0.2 m (8 in) in the 120 years from 1901-2020, and the rate of rise is accelerating.

“Human influence has warmed the climate at a rate that is unprecedented in at least the last 2000 years”

IPCC Sixth Assessment Report, August 2021

The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years.

In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years. Global surface temperature has increased faster since 1970 than in any other 50-year period over at least the last 2000 years, and glacier retreat is equally unprecedented over the same timeframe.

Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened.

Heatwaves have become more frequent with greater intensity generally with wild fires scouring every inhabited continent. Precipitation has increased in Europe and Asia with drought increasing in Africa.

“Climate change is already affecting every inhabited region across the globe with human influence contributing to many observed changes in weather and climate extremes”

IPCC Sixth Assessment Report, August 2021

Radiative forcing has increased since the beginning of the industrial age. Radiative forcing is the difference between incoming and outgoing energy in the earth-atmosphere system. It is affected by the amount of incoming solar radiation, how much is reflected back by the earth, how much is absorbed by greenhouse gases as well as the effect of and interaction of aerosols in the atmosphere (aerosols including dust and pollutants). Greenhouse gases include carbon dioxide, methane, nitrous oxide and, if you remember the big debate about the ozone layer hole over Antartica in 1985, chlorofluorocarbons (CFCs)*. As human activity is increasing the concentration of these greenhouse gases increases in the atmosphere, more heat is being retained and the radiative forcing is positive. The temperatures will rise until it gets to equilibrium.

Equilibrium Climate Sensitivity is a measure of how much the temperature will increase over the long-term as a result of a doubling of carbon dioxide. The IPCC report concludes that this is around 3°C (and could be within the range 2.5°C to 4°C).

Read the next post on The Future Climate to learn what might occur under different levels of global emissions.

Ice and snow

*A Note of Hope from CFCs. The hole in the ozone layer that came to public attention in the 1980s was a result of CFCs. Happily, the Montreal Protocol of 1987 was strengthened in 1990 to ban the use of CFCs altogether in industrialised countries by the year 2000 and by the year 2010 in developing countries. Today, the use of CFCs is outlawed by 197 countries around the world and scientists concur that the ozone layer is slowly recovering as a result. Overall, the success in addressing the ozone problem can give us hope that global environmental problems can and have been solved by humanity’s timely collective action. The evidence for wider climate action is as clear as it was in 1985 – solutions big and small are needed to address the impending crisis.