CO2 levels hit new high as climate change accelerates

Levels of the Earth’s major greenhouse gas, carbon dioxide, have hit new highs in the northern and southern hemispheres, with CO2 levels exceeding 400 parts per million, or ppm, for the first time in a northern hemisphere winter month.

Climate scientists expect that CO2 levels in the southern hemisphere will also pass the 400 ppm mark in the next 12 months. They say that to have a 50% chance of keeping human-induced average global warming below 2oC, it will be necessary to stop the release of almost all carbon dioxide before cumulative emissions reach one trillion tonnes of carbon.

But the scientists say that more than half of this huge quota has already been exceeded since the industrial revolution began in the early 1800s. At current accelerating growth rates for the combustion of fossil fuels, they estimate that the rest of the cumulative emissions will have been reached by the middle of this century.

Dr Paul Krummel and Dr Paul Fraser, atmospheric researchers at the CSIRO, Australia’s biggest research agency, note that the US government’s greenhouse gas monitoring site at Mauna Loa in Hawaii has reported that its average recorded carbon dioxide levels for February topped 400 ppm.

Another regrettable milestone

Krummel and Fraser say that although the proportion of CO2 in the atmosphere varies during the year, for the ratio to reach 400 ppm as early as February in the northern hemisphere and within the seasonal ebb and flow of concentrations “is another milestone”.

Writing in The Conversation, they say it is likely Mauna Loa’s yearly average will reach 400 ppm this year, as CO2 concentrations climb ever upwards.

Mauna Loa is one of three key sites the World Meteorological Organization uses for long-term carbon dioxide measurements, with records that started in 1956. The other northern hemisphere site is in Canada where continual records have been kept since 1975, and the third site is in the southern hemisphere, at Cape Grim in north-west Tasmania, where recording has gone on since 1976.

Tracking dramatic CO2 increases

From measurements of CO2 in the air and those in air bubbles trapped in the Arctic and Antarctic ice, scientists have been able to track the dramatic rise in CO2 from about 280 ppm before the start of the industrial era to a global average of 397 ppm in 2014 – a rise of 42% – with the greater proportion of that rise a result of human activities.

Krummel and Fraser point out that atmospheric CO2 concentrations oscillate throughout the year, reaching a peak in mid-spring in both hemispheres. This is due to the terrestrial vegetation waxing and waning along with the seasons, and is more pronounced in the northern hemisphere because of the greater land mass in that half of Earth.

During the productive growing phase from mid-spring through summer, vegetation removes considerable CO2 from the atmosphere, while it returns the gas to the air during cooler months because of ongoing respiration and vegetation decomposition.

“What we are seeing at present in the Mauna Loa February measurements are observations fluctuating around 400 ppm. These will return to sub-400 ppm levels later this year in August when the absorption by vegetation will affect northern hemisphere atmospheric CO2 levels,” the CSIRO scientists report. “However, by November 2015, Mauna Loa’s monthly average CO2 concentrations will remain above 400 ppm for the foreseeable future.”

Grim findings down south

CSIRO scientists have been able to calculate levels of CO2 in the southern hemisphere over the past 2,000 years from air trapped in Antarctic surface ice and from deeper ice cores whose depth correspond to the years in centuries past.

Krummel and Fraser say that while the northern hemisphere heads towards peak carbon dioxide levels in mid-spring, Cape Grim is heading towards the late-summer/autumn trough in concentrations. Even so, daily values are being currently recorded of around 396 ppm and the scientists expect Cape Grim to reach the 400 ppm mark for the first time during 2016, most likely around the middle of the year.

Why the south and north differ

The quantity of carbon dioxide in the atmosphere is currently rising at just over 2 ppm every year. But the northern hemisphere has higher levels because industrial and other population-based sources of carbon dioxide emissions are concentrated there.

As the CSIRO researchers point out, CO2 is one of the primary greenhouse gases while others include methane, nitrous oxide and synthetic gases, predominantly refrigerants. It is the increasing quantities of greenhouse gases in the atmosphere that is causing climate change.

“The amount of warming produced by a given rise in greenhouse gas concentrations depends on feedback processes in the climate system, such as the water vapour response. This both amplifies, by water vapour, and dampens, by cloud formation, the temperature increase due to these long-lived greenhouse gases,” they write.

“Over half of the carbon dioxide input to the atmosphere is absorbed by natural sinks in the land plants and oceans. Land and ocean carbon dioxide sinks respectively removed 30% and 24% of all anthropogenic carbon dioxide emissions over the period 2000-08. This constitutes a massive natural ecosystem service helping to mitigate humanity’s emissions.”

But unless urgent action is taken by countries around the globe to slash their release of greenhouse gases, then natural ecosystems will be unable to counter the effect, and the Earth, its lands and its oceans will continue to warm with catastrophic effects on all life.