Energy in, energy out: Earth trapping much more heat than climate models forecast
Our recent research found this imbalance has more than doubled over the last 20 years. Other researchers have come to the same conclusions.
How do you measure climate change? One way is by recording temperatures in different places over a long period. While this works well, natural variation can make it harder to see longer-term trends.
But another approach can give us a clear sense of what’s going on: track how much heat enters Earth’s atmosphere and how much heat leaves. This is Earth’s energy budget, and it’s now well and truly out of balance.
Our recent research found this imbalance has more than doubled over the last 20 years. Other researchers have come to the same conclusions. This imbalance is now substantially more than climate models have suggested.
In the mid-2000s, the energy imbalance was about 0.6 watts per square metre (W/m2) on average. In recent years, the average was about 1.3 W/m2. This means the rate at which energy is accumulating near the planet’s surface has doubled.
These findings suggest climate change might well accelerate in the coming years. Worse still, this worrying imbalance is emerging even as funding uncertainty in the United States threatens our ability to track the flows of heat.
Earth’s energy budget functions a bit like your bank account, where money comes in and money goes out. If you reduce your spending, you’ll build up cash in your account. Here, energy is the currency.
Life on Earth depends on a balance between heat coming in from the Sun and heat leaving. This balance is tipping to one side.
Solar energy hits Earth and warms it. The atmosphere’s heat-trapping greenhouse gases keep some of this energy.
But the burning of coal, oil and gas has now added more than two trillion tonnes of carbon dioxide and other greenhouse gases to the atmosphere. These trap more and more heat, preventing it from leaving.
Some of this extra heat is warming the land or melting sea ice, glaciers and ice sheets. But this is a tiny fraction. Fully 90% has gone into the oceans due to their huge heat capacity.
Earth naturally sheds heat in several ways. One way is by reflecting incoming heat off of clouds, snow and ice and back out to space. Infrared radiation is also emitted back into space.
From the beginning of human civilisation up until just a century ago, the average surface temperature was about 14°C. The accumulating energy imbalance has now pushed average temperatures 1.3-1.5°C higher.
Scientists keep track of the energy budget in two ways.
First, we can directly measure the heat coming from the Sun and going back out to space, using the sensitive radiometers on monitoring satellites. This dataset and its predecessors date back to the late 1980s.
Second, we can accurately track the build-up of heat in the oceans and atmosphere by taking temperature readings. Thousands of robotic floats have monitored temperatures in the world’s oceans since the 1990s.
Both methods show the energy imbalance has grown rapidly.
The doubling of the energy imbalance has come as a shock, because the sophisticated climate models we use largely didn’t predict such a large and rapid change.
Typically, the models forecast less than half of the change we’re seeing in the real world.
We don’t yet have a full explanation of why it has changed so fast. But new research suggests changes in clouds are a big factor.
