Anthropogenic CO₂ emissions are causing climate change, and impacts of climate change are already affecting every region on Earth. The purpose of this review is to investigate climate impacts that can be linked quantitatively to cumulative CO₂ emissions (CE), with a focus on impacts scaling linearly with CE. The reviewed studies indicate a proportionality between CE and various observable climate impacts such as regional warming, extreme daily temperatures, heavy precipitation events, seasonal changes in temperature and precipitation, global mean precipitation increase over ocean, sea ice decline in September across the Arctic Ocean, surface ocean acidification, global mean sea level rise, different marine heatwave characteristics, changes in habitat viability for non-human primates, as well as labour productivity loss due to extreme heat exposure. From the reviewed literature, we report estimates of these climate impacts resulting from one trillion tonne of CE (1 Tt C). These estimates are highly relevant for climate policy as they provide a way for assessing climate impacts associated with every amount of CO₂ emitted by human activities. With the goal of expanding the number of climate impacts that could be linked quantitatively to CE, we propose a framework for estimating additional climate impacts resulting from CE. This framework builds on the transient climate response to cumulative emissions (TCRE), and it is applicable to climate impacts that scale linearly with global warming. We illustrate how the framework can be applied to quantify physical, biological, and societal climate impacts resulting from CE. With this review, we highlight that each tonne of CO₂ emissions matters in terms of resulting impacts on natural and human systems.

Atmospheric methane levels are growing rapidly, raising concerns that sustained methane growth could constitute a challenge for limiting global warming to 2°C above pre-industrial levels, even under stringent CO₂ mitigation. Here we use an Earth system model to investigate the importance of immediate versus delayed methane mitigation to comply with the 2°C limit under a future scenario of low CO₂ emissions. Our results suggest that methane mitigation initiated before 2030, alongside stringent CO₂ mitigation, could enable to limit global warming to well below 2°C over the next three centuries. However, delaying methane mitigation to 2040 or beyond increases the risk of breaching the 2°C limit, with every 10-year delay resulting in an additional peak warming of ~0.1°C. The peak warming is amplified by the carbon-climate feedback whose strength increases with delayed methane mitigation. We conclude that urgent methane mitigation is needed to increase the likelihood of achieving the 2°C goal.

Meeting the Paris Agreement’s climate objectives will require the world to achieve net-zero CO₂ emissions around or before mid-century. Nature-based climate solutions, which aim to preserve and enhance carbon storage in terrestrial or aquatic ecosystems, could be a potential contributor to net-zero emissions targets. However, there is a risk that successfully stored land carbon could be subsequently lost back to the atmosphere as a result of disturbances such as wildfire or deforestation. Here we quantify the climate effect of nature-based climate solutions in a scenario where land-based carbon storage is enhanced over the next several decades, and then returned to the atmosphere during the second half of this century. We show that temporary carbon sequestration has the potential to decrease the peak temperature increase, but only if implemented alongside an ambitious mitigation scenario where fossil fuel CO₂ emissions were also decreased to net-zero. We also show that non-CO₂ effects such as surface albedo decreases associated with reforestation could counter almost half of the climate effect of carbon sequestration. Our results suggest that there is climate benefit associated with temporary nature-based carbon storage, but only if implemented as a complement (and not an alternative) to ambitious fossil fuel CO₂ emissions reductions.

Wetlands are the single largest natural source of methane (CH₄), a powerful greenhouse gas affecting the global climate. In turn, wetland CH₄ emissions are sensitive to changes in climate conditions such as temperature and precipitation shifts. However, biogeochemical processes regulating wetland CH₄ emissions (namely microbial production and oxidation of CH₄) are not routinely included in fully coupled Earth system models that simulate feedbacks between the physical climate, the carbon cycle, and other biogeochemical cycles. This paper introduces a process-based wetland CH₄ model (WETMETH) developed for implementation in Earth system models and currently embedded in an Earth system model of intermediate complexity. Here, we (i) describe the wetland CH₄ model, (ii) evaluate the model performance against available datasets and estimates from the literature, and (iii) analyze the model sensitivity to perturbations of poorly constrained parameters. Historical simulations show that WETMETH is capable of reproducing mean annual emissions consistent with present-day estimates across spatial scales. For the 2008–2017 decade, the model simulates global mean wetland emissions of 158.6 Tg CH₄ yr^-1, of which 33.1 Tg CH₄ yr^-1 is from wetlands north of 45°N. WETMETH is highly sensitive to parameters for the microbial oxidation of CH₄, which is the least constrained process in the literature.