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Two trillion tonnes of greenhouse gases, 25 billion nukes of heat: are we pushing Earth out of the Goldilocks zone?


Since the 18th century, people have been extracting fossil fuels from their safe storage deep underground and burning them to generate electricity or power machines.

We have now converted coal, oil and gas into more than two trillion tons of heat-trapping carbon dioxide and other greenhouse gases and added them to the atmosphere.

The current result? The average temperature at the Earth’s surface is about 1.2℃ hotter than in the pre-industrial era. That’s because adding new carbon to the world’s natural carbon cycle has created an imbalance in the amount of energy entering and leaving the Earth’s system.

Warming up the entire planet requires an extraordinary amount of extra energy. Recent research shows that over the past 50 years we have added the energy of 25 billion nuclear bombs to the Earth system.

Billions of atomic bombs to produce 1.2℃ heating – so what? It seems small, when you consider how much the temperature varies daily. (The world’s average surface temperature in the 20th century was 13.9℃.)

But almost all of this energy has so far been absorbed by the oceans. It’s no wonder we’re seeing rapid warming of our oceans.

Life on Earth is possible because we are in a good place – not too hot, not too cold.

The Goldilocks Zone

Mercury is the planet closest to the sun. It gets hot, with an average temperature of 167℃. But it has no atmosphere. That’s why the second planet, Venus, is the hottest in the solar system, at an average of 464℃. That’s because of an atmosphere much thicker than Earth’s, dense in carbon dioxide. Venus might once have had liquid oceans. But then a runaway greenhouse effect occurred, trapping truly enormous amounts of heat.

One of the reasons we are alive is that our planet orbits the Earth Goldilocks zone, just the right distance from the sun to be neither too hot nor too cold. Little of the Earth’s internal heat penetrates the cold crust where we live. That makes us dependent on another source of heat: the sun.

Read more: Global carbon emissions at record levels with no sign of contraction, new data shows. Humanity faces a monumental task

When the sun’s light and heat hit the Earth, some is absorbed at the surface and some is reflected back into space. We see some of the energy being radiated by the sun because the sun is hot and hotter objects emit radiation in the visible part of the electromagnetic spectrum.

Because the Earth is much cooler than the Sun, the radiation it emits is invisible at long infrared wavelengths. Much of this energy goes into space, but not all of it. Some gases in our atmosphere are very effective at absorbing energy at the wavelengths at which Earth emits. These greenhouse gases occur naturally in the Earth’s atmosphere and keep the planet warm enough to be habitable. That’s another Goldilocks zone.

Energy budget earth
Incoming radiation from the sun is reflected or absorbed by the earth. There is a net imbalance where more energy is absorbed than emitted by the planet and this causes warming.

And then there’s a third Goldilocks zone: recent history. All of human civilization originated in the unusually mild 10,000 years after the last ice age, when the climate in much of the world was neither too hot nor too cold.

But now we are at a very real risk of pushing ourselves beyond the comfortable climatic conditions that allowed humans to expand, farm, build and create cities.
The energy-rich fuels that made industrial civilization possible have a huge sting in their tail. Burn now, pay later. Now the bill has become clear.

How do we know this is real? Satellites measure the rate at which the Earth’s surface radiates heat. Thousands at any given time Argo robot floats dot our oceans. They spend almost their entire lives underwater, measuring heat and surface area to transmit data. And we can measure sea level with tide levels and satellites. We can cross-check the measurements between all three approaches.

Argo floats
Thousands of these robotic Argo floats monitor ocean temperatures.

Climate change: more energy coming in than going out

Greenhouse gases are powerful. You only need small concentrations to get a big effect.

We have already increased the amount of carbon dioxide in the atmosphere by about 50% and also added significant amounts of methane and nitrous oxide. This pushes our life-sustaining greenhouse effect out of balance.

A recent study suggests that the energy imbalance is equivalent to retaining about 380 zettajoules of extra heat between 1971 and 2020. (The period between 1971 and present accounts for about 60% of all emissions).

One zetta joule is 1,000,000,000,000,000,000,000 joules – a very large number!

Little Boy, the atomic bomb that destroyed Hiroshima, produced an estimated 15,000,000,000,000 joules of energy. This means that the effect of humankind’s greenhouse gas emissions over that 50-year period to 2020 is about 25 billion times the energy emitted by the Hiroshima atomic bomb.

If we’ve been retaining that much extra heat, where is it?

To date, almost every joule of extra energy – about 90% – has ended up in our oceans, particularly in the top kilometer of water. Water is an excellent heat sink. It takes a lot of energy to heat it, but we have heat. Hotter oceans are major contributors to coral bleaching and sea level rise.

temperature of the sea surface
Where’s the heat? In our oceans. This sea surface temperature map shows temperature anomalies above or below the long-term average on April 30, 2023.

It takes a long time to get that much heat into the oceans, and once it gets there, it doesn’t go away. Completely reversing global warming may not be feasible. Just to prevent the temperature from rising any further, the imbalance must be corrected and CO2 levels reduced to pre-industrial levels of 280 ppm.

If we can achieve net zero greenhouse gas emissions, we will most likely halt further global warming and carbon dioxide levels will begin to slowly decline.

Realistically, this means rapid, large-scale reductions in emissions and the use of carbon capture to offset the emissions we cannot eliminate.

Going further and cooling the planet back to a pre-industrial climate would require net negative emissions, meaning we would need to remove even more carbon from the atmosphere than any sustained emissions.

Unfortunately we are not there yet. Human-made greenhouse gas emissions are at near-record levels. But clean energy production is accelerating. This year could be the first time power emissions start to fall.

We are in a race, and the stakes are as high as possible: ensuring a livable climate for our children and for nature.

Read more: In hot water: Here’s why ocean temperatures are the highest ever recorded

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