"The outlook for Paris climate targets is bleak."

The Global Carbon Project (GCP) is an international research initiative under the Future Earth program on global sustainability. An international team of over 130 climate researchers compiles an annual report on the global carbon budget, now in its twentieth edition. From Germany, three Helmholtz Centers are contributing to the report: the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI), the Helmholtz Centre Hereon and the Karlsruhe Institute of Technology (KIT). The 2026 edition was presented at the 30th UN Climate Change Conference in Belém.

Judith Hauck serves as Deputy Section Head of the Marine Biogeochemistry Section at the Alfred Wegener Institute (AWI) and holds a professorship in Polar Biogeochemical Modeling at the University of Bremen. For the current GCP report, she coordinated the assessments concerning the ocean’s role in the global carbon cycle. In this interview, she presents the key findings and explains why the ocean’s capacity to absorb CO2 may decline in the future.

Ms. Hauck, what are the main findings of the new GCP report on the global carbon budget?

Global CO2 emissions from fossil fuel combustion are projected to reach a new record high in 2025. The anticipated emissions peak, after which levels would begin to decline, is therefore unlikely to occur. According to our forecasts, global CO2 emissions are projected to increase by 38.1 gigatons in 2025, representing 1.1% above 2024 levels. This implies that the climate target of limiting global warming to 1.5°C is now effectively unattainable. The remaining carbon budget for this target is projected to be exhausted within just four years. We also remain far from meeting the 2°C target. If current emission levels persist, the remaining carbon budget of 1,055 gigatons of CO2 will be depleted within 25 years. The international community must substantially intensify its efforts to reduce emissions.

This is concerning. Is there any positive development to report?

Indeed, despite the slightly higher emissions projected for 2025, the average rate of increase has slowed considerably in recent years. Over the past decade, it increased by an average of 0.3% per year. Over the previous decade (2005–2014), growth averaged 1.9 %. There are also signs of a trend reversal in China and India, as both countries are rapidly expanding their renewable energy capacity. Compared to previous years, emissions in these countries are now rising much more slowly, and there is even a possibility that they will stabilize. These may represent the first indicators of a peak in emissions. Thirty-five countries have reduced their CO2 emissions while maintaining economic growth, including Germany. This is twice the number of countries compared to a decade earlier. Another positive development is the decline in CO2 emissions from land-use change since the late 1990s. In the Amazon region in particular, a smaller area of forest is being cleared or burned. This illustrates the successes of environmental policies. For 2025, we project a further decline in land-use change emissions to 4.1 gigatons of CO2. Overall, these developments are positive, but progress remains insufficient to meet global climate targets.

You are an expert on the marine carbon cycle. What role do the oceans play in the global CO2 budget?

Approximately half of global CO2 emissions remain in the atmosphere, where they contribute to global warming. The remainder is absorbed by the oceans and terrestrial vegetation. Without these natural carbon sinks, global temperatures would have increased by 3 to 4 °C above pre-industrial levels, rather than 1.36 °C. For a long time, it was assumed that both carbon sinks (marine and terrestrial) absorbed roughly equal amounts of CO2. However, recent studies show that the oceans’ contribution as a carbon sink has been underestimated. Overall, the oceans have absorbed approximately 29% of all CO2 emissions over the last decade, while terrestrial vegetation has sequestered about 21%. This indicates that the ocean represents the largest natural carbon sink. 

How stable is this performance by the oceans? Will they continue to absorb almost a third of our emissions in the future?

The additional CO2 in the atmosphere is absorbed by the ocean surface layer until a new chemical equilibrium is established. As long as atmospheric CO2 concentrations continue to rise, the ocean will absorb CO2. However, chemical saturation will eventually occur, meaning that the more CO2 the ocean absorbs today, the less it will be able to take up in the future. Additionally, climate change is reducing the capacity of this carbon sink. For example, CO2 dissolves much less readily in warm water than in cold water. Consequently, a warming ocean can absorb less CO2 due to physical solubility constraints. Such climate effects are already observable. The ocean carbon sink is currently approximately 7% weaker than it would be in a world without global warming. The terrestrial carbon sink is further reduced by warming by approximately 25 %. Another significant climate effect in the oceans is that, since the 1980s, the westerly wind belt around Antarctica has intensified and shifted. This has resulted in increased upwelling of CO2-rich deep water to the ocean surface. This water cannot absorb as much CO2 because equilibrium with the atmosphere is established more quickly. If climate change continues unabated over the coming decades, these effects will become increasingly significant. Consequently, the relative carbon absorption capacity of the oceans is expected to continue declining.

Assuming that humanity succeeds in substantially reducing atmospheric CO2 concentrations in the future—for example, through novel CO2 removal technologies or large-scale reforestation— A key question is how the oceans would respond.

Ocean currents transport surface water into the deep ocean, where it circulates for several centuries before eventually returning to the surface. Much of the additional CO2 dissolved in this water will remain stored in the ocean. A new chemical equilibrium exists between the ocean and the atmosphere. Subsequently, the oceans would release the stored CO2 back into the atmosphere, thereby transitioning from a carbon sink to a carbon source.

For further information, refer to the Global Carbon Project website, which provides the complete report (English) and a summary of key findings. 

Judith Hauck, listed in the AWI expert database