Global warming has accelerated – probably because the air has become cleaner

Container ships, clouds, and sunspots have one thing in common: They are the subject of a lively debate in climate research. For several years, experts have been arguing about whether global warming has accelerated—and if so, what might be causing this.

The Earth has been warming for decades because humanity emits greenhouse gases, primarily carbon dioxide and methane from burning fossil fuels. From 1980 to 2010, the global average temperature rose by about 0.2 degrees Celsius per decade. The rate of change was virtually constant.

Since 2010, however, researchers have measured a more rapid increase: approximately 0.25 degrees per decade. Scientists have already made several suggestions as to possible causes—including both man-made and natural.

First, you have to measure the temperature. It's recorded on the Earth's surface at stations on land, but also at sea, using buoys or ships. There's a difference here. The measured data for the ocean show an acceleration of warming since 1980. The zigzag curve of temperatures is becoming increasingly steep.

When it comes to the measured data for land areas, however, you have to look twice, and even then, an increase in the rate of warming is barely noticeable. The short-term increase over the last two years is not the decisive factor, but rather the average trend.

In October 2024, an American research team still doubted that global temperature data indicated an acceleration. However, the record temperature of 2024 was not included in this study. Other research groups, however, now see clear evidence of a warming surge.

Basically, it works like this: The sun warms the Earth's surface, which then emits the heat back into space via infrared radiation. However, due to the increase in greenhouse gases, the Earth radiates less energy than it receives from the sun—the balance is disrupted. Therefore, the temperature rises.

That is why radiation measurements provide particularly valuable information on questions about faster warming: they provide information about energy gains and energy losses in the climate.

However, the energy gain from the difference between incoming and outgoing radiation has doubled over the last two decades. Scientists have determined this using satellite measurements . This doubling of the energy gain can well explain why warming has accelerated.

According to another study, this is primarily due to a decrease in cloud cover. As a result, the Earth absorbs more solar radiation. The changes in clouds are partly related to air pollution.

In recent years, air pollution has decreased in many places thanks to environmental regulations. In 2020, the International Maritime Organization issued strict rules limiting sulfate particles in the emissions of ships at sea. China has also drastically reduced its emissions of pollutants , with the country's emissions falling by 70 percent within a decade.

When fewer pollution particles are suspended in the air, they cast fewer shadows. This leads to heating. But there's a second effect, and that affects the clouds.

Many dirt particles consist of sulfates. Water vapor condenses on these particles, forming cloud droplets. The more condensation nuclei there are, the more cloud droplets form, but they are smaller. And smaller droplets reflect more light. "Air pollution caused by sulfate particles makes clouds brighter," says Richard Allan of the University of Reading in England.

Conversely, when the air becomes cleaner, the clouds become darker. Overall, the Earth's surface absorbs more sunlight and heats up more.

Climate researchers have been warning for decades that improved air quality control would further exacerbate climate change. This prediction has now apparently come true. And it is foreseeable that air pollution will continue to decline in the coming years.

Many countries around the world are trying to reduce their emissions of particulate clouds. However, one day the potential for improving air quality will be exhausted, including in South Asia and Africa. If air pollution doesn't continue to decrease, its accelerating effect on global warming will also end.

There is also a self-reinforcing effect of climate change that has to do with clouds. In the long term, clouds respond to warming in a way that intensifies the temperature rise. Certain clouds over the ocean, so-called stratocumulus clouds, are becoming fewer, says Richard Allan. This causes the ocean to absorb more solar radiation.

Solar radiation is sometimes also mentioned as an influencing factor. However, it is highly questionable whether the sun has contributed to accelerating global warming.

The star's activity does fluctuate, but its cycle length of approximately eleven years is so short that it doesn't fit well with the observed long-term acceleration of climate change. In any case, the Sun's total radiation fluctuates very little during an activity cycle.

To a certain extent, the Earth's climate can also naturally drive temperatures up or down—for example, due to changes in cloud cover. Scientists have not yet fully understood all of these internal climate fluctuations.

Scientists can use statistical methods to calculate individual natural climate fluctuations from temperature data. However, this method is not perfect. Currently, it is not possible to say with certainty whether natural fluctuations have contributed to the acceleration of warming.

Researchers currently don't know whether the acceleration will continue in the coming years. One thing seems certain: the scientific debate about the pace of climate change will continue.