The annual ZERO IN reports by the CONSTRAIN project inform on crucial scientific elements relevant to the Paris Agreement in a timely manner, providing background and context on new developments that relate to the science-policy interface.

This first report zeroes in on the remaining carbon budget as well as on projected surface warming rates over the next 20 years. Both topics are crucially important when discussing the implementation of the Paris Agreement. Estimates for these quantities will be updated annually as part of the CONSTRAIN project.

Key findings

The remaining carbon budget

Different estimates and assessments of the remaining carbon budget in the recent scientific literature have caused some confusion. Building on the methodology used in the IPCC Special Report on Global Warming of 1.5°C, this report presents a robust framework reconciling different methodological choices and provides a single best estimate for a Paris Agreement compatible remaining carbon budget.
When using the carbon budget within climate policy, policy-makers need to be aware that it is a value judgement-dependent Earth system characteristic, that is, the concept combines science with value judgements. Value judgements around warming targets and respective probabilities of meeting them, for example, are important considerations for the carbon budget’s quantification and use.

Figure 2 scientific evidence vs value judgement graphic

From the start of 2020, the remaining carbon budget is 985 Gt CO2 for limiting warming to 2.0°C with a 66% probability. To limit warming to 1.5°C with a 50% probability, it is reduced to 395 Gt CO2, and 235 Gt CO2 for a 66% probability.

Decadal warming rates

If warming continues at its current rate, we are likely to reach 1.5°C between 2030 and mid-century. The rate depends on mitigation efforts, and would likely increase to unprecedented levels if the current Nationally Determined Contributions (NDCs) or stronger mitigation efforts are not delivered. This increase would reduce the timescales available for effective adaptation, in particular for the most vulnerable.
Conversely, stringent mitigation action could reduce the rate of human-induced near- term 2020-2040 warming by up to half. This underscores the benefits of near-term emission reductions including up to 2030, the timescale of the current NDCs.
Based on our present best scientific understanding, very high near-term warming rates – double or more of those observed in the recent past – are of low likelihood. However, this does not rule them out as a possibility, emphasising the need for urgent mitigation and action towards net zero emissions to contain this risk.

Publications

Humid heat exceeds human tolerance limits and causes mass mortality

With critical physiological limits to human heat tolerance drawing ever closer, this Comment highlights the urgent need to limit further climate warming and emphasises the adaptation challenge ahead.

1.5°C-aligned 2030 and 2035 GHG emissions targets

We have produced factsheets for Australia, China, the EU, India, Indonesia and USA showing 2030 and 2035 greenhouse gas emission reduction milestones for each jurisdiction to align with the Paris Agreement’s 1.5°C goal – both including and excluding land use, land-use change and forestry.

Spatially resolved emulated annual temperature projections for overshoot pathways

This study presents a new dataset of annual temperature projections for different overshoot pathways (when the 1.5 °C of global warming limit is exceeded). The data offers a unique opportunity to study local and regional climate change impacts of a range of overshoot scenarios, including when temperature thresholds might be exceeded, and by how much.

Geological Net Zero and the need for disaggregated accounting for carbon sinks

This study in Nature show that definitions of net zero now in widespread use will not actually stop global warming. The paper demonstrates that relying on natural carbon sinks, such as in forests or oceans, to 'offset' ongoing fossil fuel emissions will lead to continued warming.

Climate Action Tracker: 2024 warming projection update

Despite an escalating climate crisis marked by unprecedented wildfires, storms, floods, and droughts, the Climate Action Tracker annual global temperature update shows global warming projections for 2100 are flatlining, with no improvement since 2021. The aggregate effect of current policies set the world on a path toward 2.7°C of warming.

Decarbonising light-duty vehicle road transport

The Climate Action Tracker provides updated 1.5ºC compatible benchmarks for the transport sector for the world as a whole and for seven individual countries; the US, EU, China, India, Brazil, South Africa and Indonesia.

Emissions Gap Report 2024: No more hot air … please!

The 2024 edition of UNEP’s Emissions Gap Report shows how much higher nations must aim. To get on a least-cost pathway for 1.5°C, emissions must fall 42 per cent by 2030, compared to 2019 levels. G20 nations, particularly the largest-emitting members, would need to do the heavy lifting.

The role of northern forests in limiting warming to 1.5°C

Northern forests are critical in the race to net zero CO₂ by mid-century. Protecting and restoring these forests, alongside steep and rapid reductions in fossil fuel emissions, is essential both to mitigating emissions and to supporting forest ecosystem services and biodiversity.

Methodology underpinning the State of Climate Action series: 2024 update

This technical note describes the State of Climate Action series’ methodology for identifying sectors that must transform, translating these transformations into global mitigation targets primarily for 2030, 2035 and 2050 and selecting indicators with datasets to monitor annual change. It also outlines the report’s approach for assessing progress made toward near-term targets and comparing trends over time.

Rapid urbanisation and climate change key drivers of dramatic flood impacts in Nepal

New study finds the devastating floods in Nepal late September were exacerbated by rapid urbanisation and climate change. The analysis found the relentless rain, which fell on saturated ground in the late monsoon, was made at least 10% heavier and 70% more likely by climate change.

LNG shipbuilding industry heading to huge oversupply

The LNG shipbuilding industry appears to be doubling down on building new carriers, ignoring the global shift to a low carbon economy and putting itself into an even worse oversupply situation than a year ago, we find in our updated report.

Seasonal forecast of two-metre temperature and precipitation in Tanzania: A hybrid cluster and point-by-point machine learning approach

Seasonal forecasting of atmospheric parameters is important in various fields including agriculture. This study tests how machine learning can complement dynamic models using Tanzanian seasonal forecasts and finds this process uses far less computational power.

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Decarbonisation targets and 1.5℃ pathways

Comment

ZERO IN on the remaining carbon budget and decadal warming rates