How humans have negatively impacted the Earth's climate

Climate change might be the most urgent issue of our day, both politically and in terms of life on Earth. There is mounting awareness that the global climate is a matter for public action.

For 11,500 years, atmospheric carbon dioxide (CO2) concentrations hovered around 280 ppm (the preindustrial “normal”), with an average surface temperature around 15°C. Since the Industrial Revolution, this level has been rising continuously, reaching 410 ppm in 2018. The geosciences, with their focus on timescales up to billions of years, are uniquely equipped to make extremely clear how abruptly industrial societies have changed and are changing the Earth’s climate.

Climate, greenhouse gases and CO2

The main engine of Earth’s climate is the sun. Our star delivers an average surface power of 342 W/m2 per year (roughly that of a hairdryer for each square meter of the planet). Earth absorbs about 70 per cent of this and reflects the rest. If this were the only climate mechanism, the average temperature would be -15°C (below the freezing point of water, 0°C). Life would likely be impossible.

Fortunately, some of the absorbed energy is re-emitted as infrared radiation, which, unlike visible light, interacts with the greenhouse gases (GHGs) present in the atmosphere to radiate heat back toward Earth’s surface. This greenhouse effect currently maintains our average temperature around 15°C.

The primary GHGs are water vapour and the much-debated CO2. Carbon dioxide contributes up to 30 per cent of the total greenhouse effect, water vapour provides about 70 per cent. CO2, though, has overall warming power that water vapour doesn’t. Water vapour in the atmosphere has a very short residence time (from hours to days) and its concentration can increase only if temperature increases. CO2 lingers in the atmosphere for 100 years and its concentration is not solely controlled by temperature. CO2 is thus able to trigger warming: if CO2 concentration increases, the average temperature, regardless of its own trend, will increase.

Carbon sinks

It is thus crucial to understand how atmospheric CO2 is regulated. Over geologic timescales (100,000+ years), volcanic gasses are the primary source of CO2, averaging 0.4 billion of tons of CO2 per year (0.4 GtCO2/y). But CO2 doesn’t just endlessly accumulate in the atmosphere. It fluxes in and out thanks to other environmental processes, and is stored in reservoirs known as carbon sinks.

The ocean, for one, contains 50 times more carbon than the atmosphere. However, CO2 dissolved in the ocean can easily be released toward the atmosphere, while only geological sinks keep CO2 away from the atmosphere on geological timescales.

A new story: the Industrial Revolution

Until the 19th century, the story of atmospheric carbon and Earth’s climate was a story of geology, biology and evolution. That story changed sharply following the Industrial Revolution, when modern humans (Homo sapiens), who probably appeared 300,000 years ago, began extracting and burning fossil fuels on a massive scale.


By 1950, the addition of CO2 to the atmosphere through fossil-fuel combustion was already proven, via the carbon isotopic signature of CO2 molecules (known as the “Suess” effect). By the late 1970’s, climate scientists observed a rapid drift toward warmer overall temperatures. The IPCC, created in 1988, showed in 2012 that the average temperature had increased by 0.9°C since 1901. That change might seem modest compared to the last deglaciation, when average temperature increased by about 6°C in 7,000 years, but it’s at least 10 times faster.

The average temperature continues to climb and natural parameters such as solar activity or volcanism can’t explain such a fast warming. The cause is unambiguously human addition of GHGs to the atmosphere and high-income countries emit the most CO2 per inhabitant.

How will our story end?

Industrial societies burnt about 25 per cent of Earth’s fossil fuels within 160 years and abruptly inverted a natural flux storing carbon away from the atmosphere. This new human-generated flux is instead adding 28 Gt of CO? per year, 50 times more than volcanoes. Natural geological sequestration cannot compensate and atmospheric CO2 keeps rising.

The consequences are imminent, numerous and dire – extreme weather events, sea-level rise, glacier retreat, ocean acidification, ecosystem disruptions and extinctions. Earth itself has survived other catastrophes. Although current warming will outpace many species’ ability to adapt, life will continue. It is not the planet that is at stake. Instead, it is the future of human societies and the preservation of current ecosystems.

While the Earth sciences cannot provide solutions to think about the necessary changes in our behaviour and consumption of fossil fuels, they can and must contribute to knowledge and collective awareness of the current global warming.

*The article was originally published by The Conversation Global Perspectives.

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