Why record temperatures in 2024 come as a surprise to scientists

More than 1.5°C above pre-industrial levels: that’s the average temperature rise measured by climate monitoring agencies for the year 2024. A record that can only be partially explained by human activities that emit greenhouse gases, and which scientists are now seeking to understand. An article by Cathy Clerbaux, Faculty of Science, in The Conversation.

Cathy Clerbaux , Sorbonne Université and Sarah Safieddine , Sorbonne Université As at the start of each year, climate monitoring agencies publish their data to quantify the average global temperature increase compared to pre-industrial times. In its January 10 press release, the European Copernicus service reported that 2024 was the hottest year since meteorological measurements began.

This figure was particularly eagerly awaited, as it means that the 1.5°C threshold, the most ambitious target in the Paris climate agreement, will be exceeded for the first time in 2024.

This year, the global average temperature measured is 15.1°C. This is rising steadily, as shown in the animation above: it is 0.12°C higher than in 2023, and 0.72°C higher than the 1991-2020 average. This is equivalent to 1.60°C above the temperature of 1850-1900, referred to as the pre-industrial level.

This increase is an average: locally, it is not the same for everyone, and may result in higher - or lower - figures depending on where you are on the globe. Most of the increase is due to human activity, which reinforces the natural greenhouse effect. But other factors also come into play, as we shall see.

Let’s take a look at why this new record has astonished scientists, and what the current hypotheses are to explain it.

The planet’s radiation balance

The first thing to remember is that without an atmosphere, the Earth’s surface would be much colder (-18°C), making it impossible for life to develop as we know it. This phenomenon, known as the greenhouse effect, is associated with the presence of so-called greenhouse gases in the atmosphere, which absorb the radiation emitted by the Earth. This is what prevents our planet from resembling Mars (too cold, tenuous atmosphere) or Venus (too hot, dense atmosphere).

When sunlight enters the atmosphere, some of it is absorbed by the ozone and oxygen naturally present in the air, protecting us from the most intense ultraviolet rays.

Another part is reflected and scattered by gases and particles suspended in the atmosphere, as well as by clouds. Volcanic activity can sometimes play an important role here, generating droplets of sulphuric acid that shield the sun’s rays.

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The remaining incident radiation can then be either reflected by the earth’s surface - a phenomenon known as albedo - or absorbed by it. The energy thus stored is then re-emitted into space in the form of infrared radiation (heat). Along the way, some of this infrared radiation is absorbed by clouds and by greenhouse gases in the atmosphere, mainly water vapour, carbon dioxide, methane, nitrous oxide, ozone and halons. This energy is then re-emitted in all directions, including towards the earth’s surface, contributing to the greenhouse effect.

The radiation balance is thus the energy entering the atmosphere, minus the energy leaving it. If this balance is disturbed, the consequence is that temperatures rise or fall.

Monitoring temperatures to distinguish weather from climate

There are "natural" variations in temperature, linked above all to the annual cycle of the seasons, depending on the latitude. Temperatures measured locally depend on the amount of solar radiation received, which varies according to latitude and season.

Monthly net radiation (in W/m² measured) by the CERES instrument aboard NASA satellites. Locations where incoming energy is greater than outgoing energy are shown in orange. Places where more energy leaves than enters are in purple. Places where incoming and outgoing energy balance out are in white/NASA The closer you get to the equator, the more solar energy you receive. Between April and September, the northern hemisphere receives the most solar energy, while the southern hemisphere benefits more during the rest of the year. With the onset of winter, net radiation becomes negative in most of the northern hemisphere and positive in the southern hemisphere.

Over a full year, there is a net energy surplus in equatorial regions and a net deficit at the poles. Over and above temperatures alone, this energy imbalance between the equator and the poles is the main driving force behind atmospheric and oceanic circulation, which redistributes this energy across the planet.

If we add to the radiation balance the thermal phenomena associated with the presence of water, known as sensible heat and latent heat (the heat required to convert a unit mass of water from one state to another, whether solid, liquid or gaseous), and also take into account internal variability (sea currents and winds), we can explain the range of temperatures measured around the globe.

The main driver of natural climate variability, which must be studied as a coupled ocean-atmosphere system, is the ENSO (El Niño Southern Oscillation) phenomenon, with its warm El Niño component and its cold La Niña component. These phenomena are the main factors in year-on-year variations, which must be taken into account when analyzing long-term trends, as well as major volcanic eruptions, which can punctually cool the climate.

In the short term, local temperature fluctuations can be explained by physical phenomena: this is "the weather". Today, we have a vast network of local measurements, both on land and at sea, supplemented by observations from instruments on board aircraft, sounding balloons and a fleet of satellites that constantly monitor the atmosphere and the earth’s surface.

This observation network is used to produce weather forecasts for the coming days, thanks to models that simulate atmospheric dynamics using mathematical equations.

Over the long term, these same observation systems play a crucial role in monitoring climate change. By accumulating observations over long periods and harmonizing them to ensure temporal consistency, they provide the essential basis for understanding climate trends and ongoing changes.

Why isn’t the planet warming up in the same way everywhere?

This year’s average figure of 1.6°C masks significant local disparities. First of all, the Earth is about 70% water and 30% land, and air heats up and cools down faster than water.

We’ve all experienced this phenomenon at the seaside, noting that water temperature is much less sensitive to weather fluctuations than air temperature. Air heats up faster than water because it has a low heat capacity, low density and, unlike water, does not take part in latent heat processes involving changes of state. As a result, almost everywhere,Öland heats up twice as fast as the sea.

Then there’s the constant transport of air and water mass from the equator to the poles, and the fact that higher temperatures increase ice melt. This phenomenon is known as "Arctic amplification".

It is also partly explained by the rapid loss of sea ice cover in this region: as the ice shrinks, the sun’s energy that would have been reflected by the bright white ice is absorbed by the ocean, causing further warming. Recent studies show that the North Pole is warming four times faster than the rest of the planet.

Partly unexplained temperature rise in 2024 - for now

In 2023, a combination of factors explained the record temperatures measured throughout the year.

In addition to the greenhouse gases that have continued to accumulate and the appearance of the natural phenomenon El Niño,

fluctuations in the solar cycle (which lasts around 11 years), which approached its maximum intensity in 2023,

the aftermath of the eruption of the underwater volcano Hunga Tonga, which released large quantities of water vapor into the stratosphere - although there is still no consensus among scientists as to the warming associated with this, and the drop in pollution in many parts of the world, including new regulations on ship fuels aimed at reducing sulfur emissions, which have a short-term cooling effect on the atmosphere.

Looking ahead to 2024 - As the El Niño phenomenon has been in a neutral phase (La Niña) since May, scientists expected temperatures to stabilize, or even decrease locally, during the second half of the year.

But that’s not what happened: temperatures remained high, particularly in the North Atlantic Ocean.

This faster-than-expected rise in surface temperatures in 2023 and 2024 is the focus of many current studies, and was the subject of a dedicated session at the American Geophysical Union (AGU), which brought together over 25,000 scientists in December 2024.

One explanation is the reduction in atmospheric pollution in recent years (good news!), whose aerosols help cool the planet by reflecting sunlight back into space.

A second possibility is the reduction in low-level cloud cover, observed in certain parts of the northern hemisphere and the tropics.

The two may be linked, as suspended particles seed low-level clouds.

Yet, according to other researchers, neither of these explanations fully explains the rise in temperatures. They suggest that global warming itself could be causing a reduction in cloud cover, creating a feedback loop that could accelerate the pace of climate change for decades to come.

There is no doubt that agencies and scientists will be closely monitoring temperature trends over the coming months, to understand local and global variations and take appropriate measures to adapt to this new reality.

Cathy Clerbaux , CNRS Research Director (LATMOS/IPSL), Visiting Professor Université libre de Bruxelles, Sorbonne Université and Sarah Safieddine , CNRS Research Associate (LATMOS/IPSL), Sorbonne Université This article is republished from The Conversation under a Creative Commons License. Read the .

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