The warming effect of long-lived greenhouse gases on our planet has increased by 41 per cent since 1990. Photo: © Shutterstock – Vadim Sadovski

Levels of long-lived greenhouse gases rose again in 2017

The level of CO2 today is similar to that 3–5 million years ago, when the temperature was 2–3°C warmer and the sea level was 10–20 metres higher than now.

Levels of heat-trapping greenhouse gases in the atmosphere have reached another new record high, according to the World Meteorological Organization (WMO). There is no sign of a reversal in this trend, which is driving long-term climate change, sea level rise, ocean acidification and more extreme weather.

Globally averaged concentrations of carbon dioxide (CO2) reached 405.5 parts per million (ppm) in 2017, up from 403.3 ppm in 2016 and 400.1 ppm in 2015. Concentrations of methane and nitrous oxide also rose.

Since 1990, there has been a 41 per cent increase in total radiative forcing – the warming effect on the climate – by long-lived greenhouse gases. CO2 accounts for about 82 percent of the increase in radiative forcing over the past decade.  

“The last time the Earth experienced a comparable concentration of CO2 was 3–5 million years ago, when the temperature was 2–3°C warmer and sea level was 10–20 metres higher than now,” said the WMO.

Carbon dioxide is the main long-lived greenhouse gas in the atmosphere. Concentrations reached 405.5 ppm in 2017, 146 per cent of the level in the pre-industrial era (before 1750). The increase in CO2 from 2016 to 2017 was about the same as the average growth rate over the last decade. It was smaller than the record leap observed between 2015 and 2016 under the influence of a strong El Niño event, which triggered droughts in tropical regions and reduced the capacity of “sinks” such as forests and vegetation to absorb CO2. There was no El Niño in 2017.

Methane (CH4) is the second most important long-lived greenhouse gas and contributes about 17 per cent of radiative forcing. Approximately 40 per cent of methane is emitted into the atmosphere by natural sources (e.g. wetlands and termites), and about 60 per cent comes from human activities like cattle breeding, rice agriculture, fossil fuel exploitation, landfills and biomass burning.

Atmospheric methane reached a new high of about 1859 parts per billion (ppb) in 2017 and is now 257 per cent of the pre-industrial level. Its rate of increase was about equal to that observed over the past decade.

Nitrous oxide (N2O) is emitted into the atmosphere from both natural (about 60%) and anthropogenic sources (approximately 40%), including oceans, soil, biomass burning, fertiliser use, and various industrial processes.

Its atmospheric concentration in 2017 was 329.9 parts per billion. This is 122 per cent of pre-industrial levels. It also plays an important role in the destruction of the stratospheric ozone layer that protects us from the harmful ultraviolet rays of the sun. It accounts for about 6 per cent of radiative forcing by long-lived greenhouse gases.

Compiled by Reinhold Pape

Source: WMO Press release, 20 November 2018

Figure: Levels of carbon dioxide over the past 800,000 years.




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