The Paris Agreement, 10 Years On: Taking Stock with Our Scientists

Ten years ago, in December 2015 during the United Nations Climate Change Conference in Paris (or COP21), 195 states adopted the ‘Paris Agreement’. This agreement paved the way towards a decarbonised and resilient world in the face of climate change.

This agreement aimed to hold ‘the increase in the global average temperature to well below 2°C above pre-industrial levels’ and pursue efforts ‘to limit the worldwide mean temperature increase to 1.5°C above pre-industrial levels’. It also set the terms of a five-year 'ambition cycle', intended to monitor progress and increase climate action ambition in order to achieve these objectives.

Each country that joined the agreement was invited to establish and regularly update their ‘Nationally Determined Contributions’ to the global effort, and to define their long-term decarbonisation strategies. Mechanisms for cooperation, technological and financial support, and monitoring progress were also an integral part of the agreement.

Today, 10 years later, with the COP30 ahead, what is the assessment of international climate action? How close are we to achieving the objectives as set in the Paris Agreement? The Belgian Climate Centre asked leading Belgian scientists to shed light on the progress made and the challenges that lie ahead.

Here is a summary of the topics covered and the experts interviewed:

1. The Atmosphere: Greenhouse Gas Concentrations | Martine De Mazière, Royal Belgian Institute for Space Aeronomy (BIRA-IASB)

2. Biosphere, Oceans and Cryosphere: The Climate Giants in Motion | Hans Verbeeck, Ghent University & Frank Pattyn, Université libre de Bruxelles (ULB)

3. Impacts on Society: Warming and Extreme Events | Xavier Fettweis, University of Liège

4. Governance: The Political and Legal Impact of the Agreement | Delphine Misonne, UCLouvain Saint-Louis Brussels

1. The Atmosphere: Greenhouse Gas Concentrations

To understand the current state of our climate, we must first look at the composition of our atmosphere. Greenhouse gases (GHGs) are the primary driver of warming. We asked Martine De Mazière, senior scientist at the Royal Belgian Institute for Space Aeronomy (BIRA-IASB), to shed light on this.

1. Since the Paris Agreement was signed in 2015, how have global GHG emissions evolved?

‘The GHGs driving climate change today are CO₂, CH₄, fluorinated gases and N₂O in order of decreasing percentage of accumulation in the atmosphere. The Paris Agreement is focusing on a reduction of the CO₂ and CH₄ emissions in the first instance.

Emissions are defined as adding GHG to the atmosphere (positive) while negative emissions or sinks are defined as uptake of GHG out of the atmosphere into the ocean or land. The difference between the emissions and the sinks is the net flux of the GHG into the atmosphere. In other words, as long as the emissions are larger than the sinks, there is a net increase of the concentration in the atmosphere.

Below we provide estimates at the global scale; it is important to remember that significant differences exist between countries/continents. And to minimize the effect of interannual variability, numbers are generally provided on a decadal basis.

CO₂

• CO₂ emissions: We distinguish between (1) fossil emissions, (2) emissions from land-use, land-use change and forestry (LULUCF, about 1/10th in amount compared to fossil emissions), and (3) an even smaller amount of emissions due to other land-use transitions and peat drainage and peat fire.

  • Global fossil CO₂ emissions are expected to further increase in 2024 by 0.8%. The 2023 emissions increase was 1.4% relative to 2022.

  • Since the late 1990s, emissions from LULUCF have shown a statistically significant decrease at a rate of the order 18% per decade.

  • Total anthropogenic emissions (fossil and LULUCF) have been stable over the last decade: zero growth rate over the 2014–2023 period, much slower than over the previous decade (2004–2013), with an average growth rate of 2.0%/year.

    
    

• CO₂ sinks:

  • The ocean net sink has been stagnant since 2016 after rapid growth during 2002–2016. The ocean CO₂ sink amounted to 26% of total CO₂ emissions during the decade 2014–2023. A slightly higher value of the sink is preliminarily estimated for 2024, which marks an increase in the sink since 2023 due to the prevailing El Niño and neutral conditions in 2024.

  • The global land net sink continued to increase during the 2014–2023 period primarily in response to increased atmospheric CO₂, albeit with large interannual variability. The land CO₂ sink amounted to about 30% of total CO₂ emissions during the 2014–2023 decade. The land sink shows a rather high interannual variability linked to El Niño and meteorological conditions causing droughts, wildfires, etc.

CH₄ (methane)

Methane has a few distinctive features compared to CO₂. Its sources are more varied, diffuse, and uncertain. A key difference is that the oceanic contribution to the global CH₄ budget is small (1%–3%), making its source estimation predominantly a terrestrial endeavour. Methane also has a shorter lifetime due to chemical removal processes in the atmosphere.

• CH₄ emissions: Global CH₄ emissions are categorised as either natural or anthropogenic.

  • Global emissions from natural and indirect sources (including wetlands, freshwater, coastal, oceanic and so-called ‘land’ sources including wildfires, permafrost, termites and wild animals) have not changed significantly between the 2000–2009 decade and the 2010–2019 decade.

  • In contrast, global anthropogenic emissions (from agriculture, waste, fossil fuels, and biomass and biofuel burning) increased by 8.2% between these two decades. They are expected to increase even faster in the early 2020s.

  • The ratio between anthropogenic and natural emissions is of the order of 1.8/1, highlighting the dominant impact of human activities on methane levels.

    
    

• CH₄ sinks: The total sink, which removes CH₄ from the atmosphere, has increased by about 3% between the 2000–2009 decade and the 2010–2019 decade.’

2. What are the current concentrations of CO₂ and other GHGs in the atmosphere, and how have these levels changed over the past decade?

‘CO₂: Currently, the mean global concentration of CO₂ in the atmosphere is 422 ppm (parts per million). Its growth rate was 2.5 ppm/year during the decade 2014–2023 with a preliminary 2024 growth rate estimate of around 2.9 ppm/year, so still increasing at an even faster rate (estimated).

CH₄: The atmospheric concentration of CH₄ is increasing. Its growth rate is accelerating: it increased from 6.1 Tg CH₄/yr in the 2000–2009 decade to 20.9 Tg CH₄/year in the 2010–2019 decade, and is expected to increase further in the early 2020s. Atmospheric CH₄ concentrations have increased from about 1800 ppb (parts per billion) in 2010 to about 1900 ppb in 2020. No significant change in the rate of increase has been observed since 2015.’

3. Can we discern any impact, however slight, from the Paris Agreement on the rate of increase of these atmospheric concentrations?

‘In 23 countries of the world with significantly growing economies, including countries in Europe and the USA, fossil CO₂ emissions decreased significantly during the decade 2014–2023. Altogether, these 23 countries have contributed about 23% of world CO₂ fossil emissions. This decrease would probably have been less or not occurred at all without the Paris Agreement.

However, for CH₄, we cannot yet see any effect: it appears that climate policies, when present, have not yet produced sufficient results to change the emissions trajectory substantially.

But, as said above, net fluxes of GHGs into the atmosphere (i.e., emissions minus sinks) should decrease significantly before concentrations in the atmosphere will stop growing and start to decrease – with a significant time lag determined by the atmospheric lifetime of GHGs which is of the order 10 years for CH₄ to 100 years or more for CO₂.

In other words, we need a substantial and fast decrease of net fluxes of GHGs into the atmosphere before we can expect to observe – with a significant delay – a decrease of GHG concentrations in the atmosphere which are causing the observed global warming.’

References:

Saunois, M., et al., Earth Syst. Sci. Data, 17, 1873–1958, 2025, https://doi.org/10.5194/essd-17-1873-2025

Friedlingstein, P., et al., Earth Syst. Sci. Data, 17, 965–1039, 2025, https://doi.org/10.5194/essd-17-965-2025.

2. Biosphere, Oceans and Cryosphere: The Climate Giants in Motion

Besides the atmosphere, the biosphere, oceans, and cryosphere (the frozen parts of our planet) are vital components of the climate system. Their responses, often slower, have very long-term consequences.

Biosphere

For the biosphere, we turned to Professor Hans Verbeeck of the Faculty of Bioscience Engineering at Ghent University.

1. What have been the dominant trends for the biosphere over the past ten years?

‘Global forests are suffering from droughts and heat waves, limiting their strength as a global carbon sink. A good example is the El Niño–related droughts and heat waves in 2023–2024 that have reduced the carbon uptake of the Amazon and Congo Basin forests to an unprecedentedly low level.’

2. How has the biosphere responded to increased atmospheric CO₂ and warming?

‘Increased CO₂ as such is fertilising plant growth; this has stimulated the annual carbon uptake of plants and forests worldwide. Therefore, the global land sink has increased proportionally to our emissions and the land biosphere is still taking up about 30% of our annual emissions. However, global warming and associated droughts put more and more pressure on this global land sink. The increased intensity of forest wildfires globally plays a key role here.’

3. What are the observed consequences or impacts of these changes on the biosphere/biodiversity?

‘In tropical forests, climate change, together with deforestation, puts an enormous pressure on the ecosystems. This causes a loss of species, and less diverse ecosystems are more vulnerable to the adverse effects of climate extremes.’

Oceans and Cryosphere

On the state of our oceans and cryosphere, Professor Frank Pattyn, glaciologist at the Université libre de Bruxelles (ULB), provides his analysis.

What have been the dominant trends for the cryosphere, such as glaciers and ice sheets, over the past ten years, and what is the current state of sea level rise?

‘Ice sheets and glaciers have continued to lose mass at an accelerating pace, and rates of sea level rise are now approaching 4 mm per year.

Exceeding 2°C will be sufficient to melt large parts of Greenland and West Antarctica, and potentially vulnerable portions of East Antarctica during the next centuries. High mountain glaciers would definitely lose more than 50% of their volume by the end of this century.

Since COP21, we know much more about how glaciers and ice sheets react to climate change thanks to detailed measurements from space and major improvements in ice flow modelling. However, the future remains grim as long as emissions are not drastically reduced. Tipping points may be reached for warming levels that are at or beyond levels of the Paris Agreement, leading to multi-metre sea level rise over the next few centuries.’

3. Impacts on Society: Warming and Extreme Events

The increase in greenhouse gases directly impacts our planet’s climate system. We spoke with Xavier Fettweis, climatologist at the University of Liège, about the tangible impacts.

1. Globally, how have Earth’s climate indicators (e.g., average temperature) evolved over the past decade?

‘In 2015, the average global temperature anomaly was approximately +1.15°C compared to the pre-industrial era. In 2024, we have reached +1.55°C, a gain of 0.4°C in 10 years, whereas over the 2006–2015 decade, the gain was approximately +0.2°C per decade. What has been particularly impressive in the last 2–3 years is the warming of the oceans, which are therefore tempering the rise in atmospheric temperature much less than before. According to the IPCC's Fifth Assessment Report (AR5) in 2013, a +1.5°C anomaly could only be reached in 2024 under the most pessimistic scenarios (RCP4.5; RCP8.5). The RCP2.6 scenario (corresponding to the Paris Agreement) did not foresee +1.5°C being reached in the 2020–2030 decade. This clearly shows that we are far from what was promised 10 years ago and that we are currently following the worst-case scenarios, suggesting a rise of > +3°C by the end of the century.’

2. What specific changes have been observed at a regional level, particularly regarding the frequency and intensity of climate extremes like heatwaves or heavy rainfall?

• ‘The July 2021 floods happened, which models (particularly the MAR-ULiège model) had suggested would occur from the 2030s onwards.

• The +40°C threshold was reached in late July 2019, whereas models (e.g. MAR) suggested this threshold would more likely be reached from 2040 onwards, even under the worst-case scenario.

• There were more than 30 heatwave days in 2018, which corresponds to the average number of heatwave days we would expect in a +3°C world.

• There has been a much stronger-than-expected slowdown in atmospheric dynamics during summer, which causes weather systems to stall. If it is a depression (low-pressure system), it leads to rain and flooding; if it is an anticyclone (high-pressure system), it leads to drought and heat. This explains why our summers are increasingly contrasted: either extremely wet (like 2021 and 2024) or dry (like 2022 and 2025).’

3. To what extent can we scientifically attribute these observed climate changes to the evolution of GHGs since 2015?

‘Clearly, all the recent extreme weather events observed in Belgium, including the July 2021 floods, were predicted by climate models that account for GHG emissions since 2015 (i.e. under scenarios like SSP3-7.0 or SSP5-8.5), but they have occurred 10 to 20 years earlier than projected. In other words, what we are observing now is what the models predicted for 2030–2040, and this trend applies to all of Europe, which is currently warming much faster than models predicted. This acceleration in warming in Europe is largely linked to a faster-than-expected slowdown in atmospheric dynamics and an improvement in air quality (which is a good thing in itself). It is quite paradoxical, but air pollution (aerosols) reduces sunlight and thus cools the climate. Less pollution means more sun and a runaway effect on temperatures, particularly in summer.’

4. Governance: The Political and Legal Impact of the Agreement

The Paris Agreement is a political and legal instrument presented as a game-changer. To what extent has it influenced legislative frameworks? We have asked of Delphine Misonne, Senior Research Associate at FNRS, Professor at UCLouvain Saint-Louis Brussels, and Head of CEDRE, an environmental law specialist. 

1. With 10 years of hindsight, what political assessment can be drawn from the Paris Agreement? Can we talk about a major shift in international climate action?

‘Can a series of weak treaties on climate change be transformed into a success story? The answer is in the question. Facts show the difficulty for the Paris Agreement to deliver on its best promises. Meeting challenging global climate goals through the spontaneous accumulation of adequate nationally determined contributions is an experiment on a world scale. The formula was not inspired by human and social sciences research results which could have provided the ideal equation (on conditions and deadlines) for guaranteeing a successful outcome. One of the problems with the Paris Agreement is that it does not provide, in any way, the level playing field (with due respect to the respective capacities) that proved to be so deficient at the time of the Kyoto Protocol. On the contrary, efforts and pledges have never been as asymmetric as they are today. After ten years, the Paris Agreement now proves to be more a continuation (of the first global decisions made in the nineties) than a major shift in international climate action.

A major shift would have been to define a clearer status for the protection of the climate system as such; be clearer and more concrete on the global targets as far as consumption/production patterns and energy sources are concerned; identify the missing guests (invite the climate system and human rights to the table of negotiations). Nevertheless, in the meantime, international law on the climate responsibility of States has considerably evolved, transforming the Paris Agreement into a piece of a much larger puzzle. The clarity and severity in the interpretations that were recently given by the highest international Courts (International Court of Justice, International Tribunal of the Law of the Sea, Inter-American Court of Human Rights), in their respective advisory opinions, brings in the potential for a major shift. It is now made clear that all States have, under customary law (and thus even if they withdraw from the Paris Agreement), a duty to co-operate with each other in good faith to prevent significant harm to the climate system and other parts of the environment, which requires sustained and continuous forms of co-operation by States when taking measures to prevent such harm; and all States have obligations under international human rights law to respect and ensure the effective enjoyment of human rights by taking necessary measures to protect the climate system and other parts of the environment.’

2. What concrete impact has the Paris Agreement had on national legislation and regulatory frameworks around the world?

‘The text of the Paris Agreement does not specify that pledges need to be consolidated in dedicated legislation and regulatory frameworks at the national level. But this is an evident and necessary follow-up, when Parties want to show how serious they are with their own commitments. It also offers political support to the diplomatic action, and is the optimal way to craft the necessary infrastructure for a well-thought-out, inclusive, just and transparent governance of the climate challenge. This is why, under pressure from civil society, dedicated climate legislation flourished after the entry into force of the Paris Agreement, at least in Western Europe. At European Union level, the Green Deal also needed to be translated into concrete legislative acts, including a European Climate Law on common goals at the scale of the European Union. But only after a few years, the content of several forward-looking pieces of legislation is already under threat. Keeping the ambition in the long term is a daunting challenge for climate action, quite unique in legislative history. This is where climate litigation comes into play. Lawsuits defying national climate actions, even where States have adopted dedicated climate laws, certainly tend to push the ambition of the lawmaker forward, and clarify the duties of the lawmaker. Again, the Paris Agreement alone would not have delivered without that kind of public pressure (which only exists where access to justice on climate issues is made possible).’