Global warming reaches record speed, but what’s causing it? † NOW

We know that the earth is getting warmer from direct measurements of the temperature with thermometers. This shows that the average temperature on earth is currently rising about 0.2 degrees every ten years.

But there are other ways to measure that warming. An important one is using satellites. They can measure how much energy the Earth loses to space, and are fairly accurate in that regard nowadays.

Satellites show that Earth does not lose heat

If the earth radiates less energy than it receives from the sun, that means the earth is holding net energy; a sign that the atmosphere and oceans are getting warmer.

This is called the radiation balance, and it is expressed in Watts per square meter of Earth’s surface, measured at the top of the atmosphere. If the value is positive, the earth is getting warmer. That has been the case for decades. Between 1960 and 2015, the earth gained an average of about 0.4 Watt of energy per square meter, causing the temperature to rise almost one degree over the same period.

Between 2017 and 2022, that was already an average of 1.19 Watt per square meter. And that ‘radiation imbalance’ has now skyrocketed to 1.64 Watt per square meter measured over twelve months between March 2021 and February 2022, warns climate expert Leon Simons.

This suggests that warming is accelerating significantly. But it can also be a temporary spike. So what exactly is the cause?

Less sun reflection above shipping routes

Simons and the renowned American climatologist James Hansen are investigating one possible explanation for faster warming: a very sharp decrease in sulfur emissions from shipping, by adapting the fuel since 2020.

Sulfur compounds are bad for the environment and health, but they do have a cooling effect because they reflect sunlight. And according to Simons, this net reflection of sunlight has indeed decreased in recent years due to a combination of causes.

“I think the effect of reduced sulfur emissions from shipping is significant because satellites see that in areas where many ships are sailing, the decrease in reflected sunlight is strongest.”

Concentration of greenhouse gases is also accelerating (slightly)

What about the concentration of greenhouse gases? These greenhouse gases trap heat in the atmosphere. That is ultimately the reason that the Earth’s radiative balance is ‘positive’; greenhouse gases trap some of the sun’s energy within the atmosphere. The outgoing radiation to space is therefore smaller, which is measured by satellites.

But isn’t that acceleration in the radiation difference due to a faster increase in the concentration of greenhouse gases? That is also possible, but then we should also be able to measure it. The concentration of CO2, the most important greenhouse gas, continues to increase every year. There has also been a slight acceleration visible over the years, but certainly not doubling in the very short term.

The increase in the concentration of the greenhouse gas methane, after CO2, the most important cause of warming, has been accelerating somewhat more clearly in the past ten years. But even that increase is too weak to explain the satellite observations.

Another explanation: more upwelling from the deep sea

That leaves an important candidate: the oceans. They extract a lot of heat from the atmosphere. Exactly how much varies from year to year. In years when extra deep, cold ocean water wells up to the surface, the atmosphere can lose more heat in the water, so less energy radiates to space.

This is the case during the natural climate change La Niña, when a lot of heat disappears in the water of the Pacific Ocean. And that is indeed what is happening in 2022.

Yet we have had La Niña more often in recent years, without this leading to record measurements from climate satellites. So how should we explain it then? As is often the case in the complex climate system: by adding up all those factors.

At the moment, extra heat is being lost in the ocean water and extra heat is being retained by the accelerated rising concentration of greenhouse gases.

And the human-induced cooling of the earth, due to sun-reflecting sulfur particles, is actually decreasing in the meantime. If you ask Simons, that last factor is the strongest. In that case, we will eventually see this again with ordinary thermometers, as an acceleration in global warming.