Joeri Rogelj

Since the adoption of the 2015 Paris Agreement, countries have put forward emission reduction pledges which altogether would keep global warming below 2°C. Policy implementation lags these pledges but has nevertheless markedly impacted the outlook. Policy implementation and the shifts in, for example, technology costs they engender have lowered end-of-century global warming prospects since 2015 by about 1°C, yet still imply exceeding 2°C and 1.5°C of global warming by a large margin.

The Paris Agreement establishes that Parties’ Nationally Determined Contributions (NDCs) will represent a progression, a country’s highest possible ambition (HPA), and an equitable contribution to the collective long-term temperature goal. However, the agreement provides no guidance on how the HPA standard should be interpreted, assessed, or operationalised in practice. In response to this gap, initial scholarship has proposed frameworks to interpret HPA based on the treaty’s text and the wider international environmental law context, which includes the 2025 Advisory Opinion on Obligations of States in Respect of Climate Change from the International Court of Justice. Altogether, this has clarified how HPA is part of states’ due diligence obligations and sets the stage for the next challenge: how can the theoretical understanding of HPA be practically applied to assess whether NDCs reflect this norm? Here, we build on existing reporting obligations under the United Nations Framework Convention on Climate Change (UNFCCC), emerging approaches reflected in the UNFCCC NDC synthesis report, and best practices from the research community. We describe expectations for the process, conduct, presentation, and documentation of NDCs of HPA. These expectations provide qualitative benchmarks and criteria for assessing whether Parties have undertaken a rigorous and reasoned process to identify their highest possible mitigation efforts, and whether the evidence provided is sufficient to make that reasoning open to scrutiny. This promotes transparency and consistency in the formulation and assessment of NDCs, whilst intentionally not assessing targets against quantified mitigation benchmarks. Finally, the proposed expectations are contextualised and illustrated with examples from the latest round of NDC submissions. This practical translation of theoretical obligations can serve as a starting point for a deeper policy conversation on how HPA of NDCs can be evidenced, assessed or challenged.

Literature estimates of natural methane emissions, particularly from wetlands, have a wide range of uncertainty. Meanwhile, few Earth System Models (ESMs) explicitly model wetlands as a potential source of methane. As a result, Simple Climate Models that aim to emulate the behaviour of ESMs have little to constrain their present and future contributions. MAGICC, as of version 7.5.3, fixed natural methane concentrations as constant after the historical period. Two studies that model wetland methane emissions over the 21st century both find a relationship between those emissions and global temperature, though disagree on the extent of this temperature sensitivity. An updated version of MAGICC has been created that uses this evidence to include a linearised representation of the relationship between wetland methane emissions and global temperature. The temperature-sensitivity parameter in this relationship has been parametrised in a way such that its distribution encompasses the uncertainty in both modelling literature and carbon budget studies, reflecting the currently high degree of uncertainty in wetland emissions. Our results show how incorporating a temperature feedback in methane emissions leads to both higher temperature projections for all scenarios used here, and a widening of the uncertainty in global temperature response.

Integrated assessment model scenarios informing global climate ambition typically treat fairness and cooperation as external to the modelling process, reinforcing a perceived tension between fair effort-sharing and the urgent collective action needed to minimise exceedance of 1.5 °C. Here we introduce a generalisable framework that embeds principle-based fair-share considerations directly into the scenario generation process. This framework produces scenario variants that reconfigure transition pathways while preserving global peak and end of century climate outcomes. We explore this space across allocation approaches, transfer limits, and cooperation mechanisms, and propose a new lowest-feasible transfer scenario variant to bound it. We examine the integrated system shifts this new variant implies, and contrast it with the common exploration of fairness through ex-post assessments and unlimited financial transfers. We observe that the renewables deployment and electrification trajectories remain largely unchanged across all fair-share variants, while restricting transfers to lowest-feasible volumes accelerates global fossil decline. This effect prevails even when interregional cooperation is focussed on financing carbon dioxide removal with geological storage, where higher-responsibility regions’ fair-share obligations remain met primarily through domestic gross emissions reductions. Against a globally cost-effective 2 °C (>67%) scenario that reduces discounted consumption by approximately 0.8% relative to the no-new-policy reference scenario, integrating fair-share considerations raises this to at most 1.3% when transfers are restricted to lowest-feasible volumes. Notably, this aggregate is distributionally progressive, where lower-responsibility regions gain relative to the cost-effective source scenario. This deepened solution space persists even under a stringent 1.5 °C goal, though with differentiation increasingly settled through finance, and reveals dynamics that emerge only when scenario generation itself accounts for differentiated responsibilities and capabilities. Ultimately, we find that a ‘gross emissions reductions first’ strategy among higher-responsibility regions generates system-wide benefits, releasing pressure on lower-responsibility regions to collectively meet climate goals.

The Paris Agreement’s ‘well below 2 °C’ target is often interpreted as holding warming below 2 °C with a probability in the 66–90% range. We argue that such interpretations are not future-proof as decreasing uncertainties with time will allow temperatures to approach 2 °C later in the century. We show how referring to median warming can resolve this issue and illustrate how past probabilistic target interpretations would translate to a median 1.65–1.8 °C warming limit range.

Biodiversity loss is expected to escalate with every increment of global warming. Simultaneously, land-intensive climate change mitigation strategies, such as afforestation and bioenergy, may further compound biodiversity loss. So far, the magnitude of these two drivers has not been compared in the context of temperature overshoot, meaning the temporary exceedance of a targeted global warming limit. By combining spatial data on climate refugia (areas sheltering biodiversity from climate change), bioenergy cropland, and forestation for multiple cost-effective scenarios with varying levels of climate action and overshoot, we illustrate how both warming and mitigation affect today’s climate refugia across five integrated assessment models. Decisive climate action, compatible with limiting warming to 1.5 °C, reduces the combined loss of today’s climate refugia due to warming and mitigation-related land-use change by more than 50% compared to current climate policies, outweighing potentially negative implications of mitigation at the global level by limiting the magnitude and duration of warming above 1.5 °C. We observe notable differences across regions and the considered model frameworks. Overshoot implications strongly depend on the underlying biodiversity recovery assumptions.

In a rapidly changing climate, evidence-based decision-making benefits from up-to-date and timely information. We track twelve key sets of indicators of the state of the climate system, closely following Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment report (AR6) methods, to produce our fourth annual publication. One of the indicators, the Earth's energy imbalance (EEI) provides a crucial integrative measure of the overall heating of the planet and the pace of climate change – this has more than doubled since the 1976–1995 period. A newly added indicator of temperature extremes, the number of days experiencing marine heatwaves, has more than tripled between 1991 and 2025. For the 2016–2025 decade average, observed warming relative to 1850–1900 was 1.26 [1.13 to 1.36] °C, of which 1.24 [1.0 to 1.5] °C was human-induced. Human-induced warming reached 1.37 °C relative to 1850–1900 in the year 2025, increasing at a rate of 0.27 [0.2–0.4] °C per decade over 2016–2025. This high rate of warming, which matches the all-time high seen last year in the instrumental record, was caused by a combination of greenhouse gas emissions being at an all-time high of 54.6 ± 5.5 GtCO ₂ e yr ⁻¹ over the last decade (2015–2024), as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that CO ₂ emission growth is slowing. The continuation of these annual updates could track decreases or increases in the rate of human influence and climatic changes presented here, reflecting the outcomes of societal choices during the critical 2020s decade. The data presented herein can provide a useful reference point for the drafting of the IPCC seventh assessment report. In total, we employ analysis from over 40 global datasets (https://doi.org/10.5281/zenodo.20499280, Smith et al., 2026a). Future monitoring of these indicators, such as ocean and satellite measurements of the Earth's energy imbalance, are threatened by geopolitical and public funding decisions. Our ability to consistently track many of the indicators requires the continuity of observation programs and coordination mechanisms, including the Global Climate Observing System (GCOS) program, that enable their effective integration and use.

As global warming approaches 1.5 °C, the term ‘overshoot’ is becoming increasingly prominent in climate science, policy, and public communication, but the use of this term remains inconsistent and often ambiguous. This article traces the history of the term in ordinary language and science-policy contexts, showing how specialized uses such as ‘temperature overshoot’ or ‘overshoot pathway’ have diverged from the common meaning of exceeding a target or limit. We provide practical guidance for clearer communication, recommending that authors respect the ordinary meaning of overshoot and specify where insights refer to exceedance, peak warming, decline, or return below a given warming level, while avoiding short-hand terminology where it risks misunderstanding.

Direct Air Capture (DAC) is widely considered essential for achieving net-zero and net-negative emissions, yet its potential to scale to climate-relevant levels remains uncertain. Here we show that the future deployment of DAC depends primarily on early capacity expansion and growth dynamics rather than on long-term demand targets alone. Using a probabilistic technology diffusion model informed by historical analog technologies and uncertain future demand, we explore a wide range of possible global DAC deployment pathways to 2050. We find that if DAC follows growth trajectories similar to ammonia synthesis technologies and liquefied natural gas, deployment is likely to remain at the megaton scale by mid-century. However, gigaton-scale deployment becomes plausible under rapid growth and strong early policy support. Our results identify short-term capacity expansion as the most effective lever for accelerating DAC deployment and highlight the critical importance of timely policy action to avoid overreliance on future large-scale carbon removal.