By Chris Jones (MOHC, UK), January 2019.

A CRESCENDO co-organized workshop on carbon budgets brought together eight IPCC lead authors and 10 international carbon cycle scientists to make progress towards a major update of the remaining carbon budget to achieve the Paris Agreement Climate targets of 1.5 and 2 degrees. CRESCENDO partners from UK, France and Germany were amongst participants who made substantial progress towards this important goal.

Since IPCC AR5 quantified the carbon budgets compatible with a range of climate targets (Figure 1, reproduced from IPCC AR5, https://www.ipcc.ch/report/ar5/wg1/summary-for-policymakers/) – the so-called TCRE: the transient climate response to cumulative carbon emissions – there has been active research into improving and refining these estimates to reduce their uncertainty.

Figure 1. Relationship between cumulative carbon emissions, and resultant global temperature rise. Reproduced from IPCC AR5 Summary for Policy Makers, figure SPM-10.

The recent IPCC Special Report on 1.5 degrees (https://www.ipcc.ch/sr15/) made substantial advances in the methodology which led a significant increase in estimates of remaining budgets. However, this update was still based on old CMIP5 Earth System Models and therefore a major update with CMIP6 results is required and will be presented in IPCC’s forthcoming Sixth Assessment report (AR6) in 2021. Advances to European ESMs made within the CRESCENDO project will play a crucial part of this major update, and several CRESCENDO scientists are lead authors in the IPCC report.

“Now is a crucial time for us to exploit the wealth of new information coming from new ESMs” explained workshop co-organizer, and CRESCENDO RT2 lead, Chris Jones. “Developments during the course of CRESCENDO, and the simulations we have performed and delivered to the Earth System Grid will enable a step-change in our ability to quantify carbon budgets to remain within the Paris Climate Agreement targets.”

The workshop, co-organized by the Global Carbon Project (http://www.globalcarbonproject.org) followed the IPCC Working Group 1’s 2nd lead author meeting (https://www.ipcc.ch/event/second-lead-author-meeting-of-the-ipcc-working-group-i-sixth-assessment-report/) in Vancouver, Canada. By its co-location it minimized additional travel for workshop participants and maximized attendance by eight IPCC lead-authors from across five chapters, ensuring the new science emerging from CRESCENDO can have maximum influence on key IPCC findings.

Joeri Rogelj, workshop co-organiser and a coordinating lead author of the IPCC Special Report on 1.5 degrees, commented on particularly exciting and stimulating interactions at the workshop. “We were able to identify a couple of key research directions” he added.

The outcomes of the workshop are planned to lead to multiple high-profile publications in areas including a formalized framework to understand the components affecting the remaining carbon budget, a method of propagating and reducing uncertainty in climate and carbon cycle feedbacks to carbon budgets (building on CERSCENDO RT2: Model Evaluation work, and deliverables D4.1: Evaluation of terrestrial processes, and D8.4: Constraining allowable carbon emissions necessary for achieving 1.5 and 2C), and new ways to consider non-CO2 forcing and the role of negative emissions.

Papers submitted ahead of 31st December 2019 can be included for citation in the WG1 AR6 assessment.

Workshop participants are very grateful to local host, and IPCC carbon cycle lead author, Kirsten Zickfeld, for the hard work which enabled an extremely productive and enjoyable workshop.