Why the temperature record in 2024 is a surprise for scientists

More than 1.5°C compared to the pre-industrial era: this is the average increase in temperatures measured by climate monitoring agencies for the year 2024. A record which can only be partially explained by the activities humans emit greenhouse gases and which scientists are now seeking to understand.

Like every beginning of the year, climate monitoring agencies publish their data to quantify the average increase in temperature on a global scale compared to pre-industrial times. In its press release dated January 10, the European Copernicus service indicates that 2024 was the hottest year since meteorological measurements existed.

This figure was particularly expected, because the threshold of 1.5°C, which is the most ambitious objective of the Paris climate agreement, is exceeded for the first time in 2024.

Rising Temperatures/NASA Scientific Visualization Studio (reload page to restart animation)

This year, the measured global average temperature is 15.1°C. This is increasing regularly, as shown in the animation above: it is 0.12°C higher than that of 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 the location on the globe. It comes, for the most part, from human activities which reinforce the natural greenhouse effect. But other factors also come into play, as we will see.

Let’s examine together why this new record surprised scientists and what the current hypotheses are to explain it.

The planet’s radiation balance

We must first remember that without an atmosphere, the Earth’s surface would be much colder (-18°C), making the development of life as we know it impossible. 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 allows our planet not to resemble Mars (too cold, thin 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 sulfuric acid which shield solar radiation.

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The remaining incident radiation can then either be 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 present in the atmosphere, mainly water vapor, 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 radiative balance is thus the energy which enters the atmosphere from which we deduce the energy which leaves it. If this balance is disturbed, the consequence is that temperatures increase or decrease.

Monitor temperatures to distinguish weather from climate

There are “natural” variations in temperatures which are above all linked to the annual cycle of the seasons, depending on the latitudes. The temperatures measured locally in fact depend on the quantity of solar radiation received, which varies according to latitude and the seasons.

Monthly net radiation (in W/m² measured) by the CERES instrument aboard NASA satellites. Places where energy in is greater than energy out are orange. Places where there is more energy going out than energy coming in are purple. The places where the amounts of incoming and outgoing energy balance are white/NASA (reload the page to restart the animation)

The closer we get to the equator, the more solar energy received. 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, the net radiation becomes negative in most of the northern hemisphere and positive in the southern hemisphere.

Over a full year, we therefore observe a net surplus of energy in the equatorial regions and a net deficit at the poles. Beyond just temperatures, this energy imbalance between the equator and the poles constitutes the main engine of atmospheric and oceanic circulation, which redistributes this energy across the planet.

If we add to the radiative balance the thermal phenomena linked to the presence of water, known as sensible heat and latent heat (this is the heat that must be supplied to a unit of mass of water for the move from one state to another, solid, liquid or gas), and also by taking into account internal variability (marine currents and winds), we manage to explain the range of temperatures measured all 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 of variation from one year to the next, which must be taken into account when analyzing long-term trends, as well as for major volcanic eruptions, which can occasionally cool the climate.

In the short term, local fluctuations in these temperatures can be explained by physical phenomena: it is “the weather”. Today, we have a vast network of local measurements, carried out both on land and at sea, supplemented by observations from instruments on board aircraft, sounding balloons and a fleet of satellites which constantly monitor the atmosphere and the earth’s surface.

This observation network makes it possible to produce weather forecasts for the days to come using models that simulate the dynamics of the atmosphere using mathematical equations.

In the long term, these same observation systems play a crucial role in monitoring climate change. By accumulating observations over long periods of time 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?

The average figure of 1.6°C measured this year masks significant local disparities. First we must take into account that the Earth is made up of approximately 70% water and 30% land, but air heats up and cools down more quickly than water.

We have 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 does not participate in processes requiring latent heat involving changes of state, unlike air. ‘water. As a result, almost everywhere, the land is warming twice as fast as the sea.

Next, we must take into account the constantly occurring air and water mass transports from the equator to the poles as well as the fact that higher temperatures increase ice melting. This phenomenon is known as “Arctic amplification”.

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

A partly unexplained rise in temperatures in 2024 – for now

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

What about 2024? As the El Niño phenomenon has shifted to a neutral phase (La Niña) since May, scientists expected temperatures to stabilize, or even decrease locally, during the second half of the year.

However, this is 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 at the center of many current studies, and was the subject of a dedicated session at the American Geophysical Union (AGU), which brought together more than 25,000 scientists in December 2024.

• A first explanation would be the reduction, in recent years, of atmospheric pollution (good news!), whose aerosols contribute to cooling the planet by reflecting sunlight into space.

• A second avenue would be the reduction in low-altitude cloud cover, observed in parts of the northern hemisphere and the tropics.

The two could be linked, because suspended particles seed low-level clouds.

However, according to other researchers, neither explanation fully explains the rise in temperatures. These suggest that global warming itself could be causing a reduction in cloud cover, creating a feedback loop that could accelerate the rate of climate change for decades to come.

There is no doubt that the evolution of temperatures over the coming months will be closely monitored by agencies and scientists, to understand local and global variations and take appropriate measures to adapt to this new reality.