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The annual ZERO IN reports by the CONSTRAIN project provide information on scientific topics that are crucial to the Paris Agreement, including background and context on new developments at the science-policy interface. This includes new insights into the complex processes represented in climate models and what they mean for temperature change and other climate impacts over the coming decades.

These advances in climate modelling are particularly relevant when it comes to the heart of the Paris Agreement: the Long-Term Temperature Goal (LTTG), put in place to avoid the most catastrophic impacts of climate change. The LTTG calls for “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels”.

The latest generation of climate models (CMIP6) is improving our understanding of the climate system and where global temperatures are heading, including when we might pass the 1.5 or 2 ̊C thresholds, as well as the mitigating actions that can help us to avoid doing so. However, the new model results require careful interpretation.

We also need to understand how temperature change is measured in the context of the LTTG, to both assess how global temperatures have changed to date, and to use the models effectively in making decisions that affect our climate future. This is particularly important given the economic and societal choices the world faces in the light of the COVID-19 pandemic.

This year’s ZERO IN report therefore focuses on the new CMIP6 climate models and the science behind the LTTG, highlighting how improved understanding in both areas can help us to better plan for what lies ahead. In particular, we find that whilst the effect of COVID-19 on climate has so far been negligible, a green recovery could profoundly alter the trajectory of climate change over the next two decades. Our findings also reaffirm the importance of stringent near-term emission reductions and reaching net-zero CO2 emissions by 2050 to get the world on a 1.5°C pathway.
In addition, we provide our annual update on the remaining global carbon budget. This includes an estimate for the budget remaining from the start of 2021 alongside further context on the use of carbon budgets in national and regional policy.

Key findings

HOW MUCH WARMING THE NEW CLIMATE MODELS PROJECT

• Some of the latest climate models (CMIP6) show that, if atmospheric CO2 concentrations double from pre- industrial levels, their temperatures rise more than expected from other lines of evidence. The higher values are thought to be largely the result of changes to how the models represent complex cloud processes.
• The range of CMIP6 projections can be narrowed down by comparing them with observations of recent temperature change, using a method called “constraining”. This shows that the CMIP6 models with higher future temperature projections also overestimate past temperature rise, suggesting that these projections are also too high.
• Overall, there is little evidence for the stronger future warming projected by some CMIP6 models, and the constrained CMIP6 range is consistent
• with previous model generations, indicating where global temperatures are heading, depending on the emissions pathway we follow.

UNDERSTANDING WHERE WE ARE IN TERMS OF THE PARIS AGREEMENT LONG-TERM TEMPERATURE GOAL (LTTG)

• The Paris Agreement reflects global, human-made long- term temperature change that excludes the short-term natural variability in the climate system. Exceeding 1.5°C warming during one or more years as the result of year- to-year variability therefore does not mean that the Paris Agreement LTTG has been reached or exceeded.
• Measuring where we are now with respect to the LTTG means using the same approach that was used to set it, following the best available science at the time as set out in the IPCC Fifth Assessment Report (AR5). This includes looking forward from a modern reference period (1986- 2005), and so scientific advances in establishing how temperatures changed before this time will not affect our trajectory towards the 1.5°C limit.
• Overall, reaching or exceeding 1.5°C warming in a single year, month or location does not mean that the LTTG has been breached, as long as human- made warming still falls below 1.5°C. It is unlikely that human-made warming will reach 1.5°C above pre- industrial levels in the next decade.

COVID RECOVERY, NEAR-TERM WARMING AND MEETING THE PARIS AGREEMENT

• Integrating hard and fast climate action with COVID economic recovery packages could, over the next 20 years, slow down human-induced global warming by up to half the rate we have experienced since 2000, giving us vital time and space to adapt to future climate impacts.
• This “strong green recovery”, investing just 1.2% of GDP in green technologies and industries, whilst refusing to bail out fossil fuel companies, could also cut the total amount of warming by 2050, putting us back on track to stay within the LTTG’s 1.5°C limit.
• In addition, this approach would get us on the path to net-zero, where, facilitated by decisive political action that leads to structural economic change, everyone can play their part in ensuring that, as a global community, we avoid the most dangerous climate impacts.
• Such a green recovery is urgently needed as the carbon budget continues to be depleted despite a record fall in annual CO2 emissions from 2019 to 2020. We assess the remaining carbon budget for staying below 1.5°C to be 355 Gt CO2 (50% probability).

The EU-funded CONSTRAIN project is a consortium of 14 European partners, including Climate Analytics, tasked with developing a better understanding of global and regional climate projections for the next 20-50 years.