Leaders must agree on how to measure 1.5 °C of global warming

A team of Met Office scientists has emphasised that there is no formally agreed way of defining the current level of global warming relevant to the Paris Agreement. They have proposed a solution.

The team includes Professor Jason Lowe at the School of Earth and Environment. Ahead of COP28, they published the article ‘Approaching 1.5 °C: how will we know we’ve reached this crucial warming mark?’ in Nature. 

In 2015, the Paris Climate Agreement saw world leaders agree to take measures to limit planetary warming to 1.5 °C. They aimed to achieve this through actions such as lowering emissions and relying less on fossil fuels.

However, as the researchers write in their article, “there is at least a 50% chance that long-term global warming will overshoot 1.5 °C in the next decade.” 

Tracking the planet’s temperature 

Tracking the planet’s temperature is important because it allows scientists and policymakers to assess how effective preventative measures are. 

Despite this, the Paris Statement did not contain a method for defining levels of global warming. There are regular ups and downs in global temperatures, so agreeing on the specifications of a 1.5 °C temperature rise is necessary.

Professor Richard Betts of the Met Office and the University of Exeter is the paper’s lead author. He said: 

“Clarity on breaching the Paris Agreement guard rails will be crucial. Without an agreement on what actually will count as exceeding 1.5 °C, we risk distraction and confusion at precisely the time when action to avoid the worst effects of climate change becomes even more urgent.” 

The researchers argue that a method must be agreed upon that filters out natural climate cycles so that scientists and policymakers can be sure of the meaning of the measurements, forecasts and related actions. 

How should it be measured? 

“Researchers and the policy community need to agree urgently on a metric for determining the current level of global warming for policy purposes,” the researchers write. 

In the article, they discuss the most common and newer ways of measuring global temperatures and how they have both benefits and flaws. 

For example, measuring the temperature over long periods helps to account for anomalies, but means that years could pass between a breach of 1.5 °C and its formal recognition. 

In their article, the researchers propose a new technique that would keep the benefits of a long-term measurement but that is also: 

• consistent with the Intergovernmental Panel on Climate Change (IPCC)’s definition of 1.5 °C of warming 
• able to recognise the current level of global warming without delay
• future-proof.

They suggest blending a past 10-year average with climate model projections or forecasts to achieve this. 

This ‘current global warming level’ would provide an instant indicator of current warming. 

The researchers call for the international community to recognise the need for an agreed metric for passing 1.5 °C. 

They say: 

“We encourage the IPCC to tackle this issue in a Special Report ahead of its seventh assessment report (AR7), which is not expected to be published until about 2030 — by which time, global warming might already have exceeded 1.5 °C or be close to doing so.” 

Read ‘Approaching 1.5 °C: how will we know we’ve reached this crucial warming mark?’ in Nature.