NEWS BRIEFING and EXPERT REACTION: We may have already passed 1.5°C of global warming

At first look, this single new study seems to say that the IPCC radically underestimated warming. However, it is studies exactly like this that highlight the merit of the IPCC, in which hundreds of scientists comb through thousands of scientific studies to distil robust findings. 

A single new paleo record off the coast of Puerto Rico is a valuable addition to the large evidence of warming. But it is just that, one study among hundreds. The IPCC's findings still stand strong.
 
We are close to 1.5C warming and there is no reason for complacency on the path towards net zero emissions. We can keep our established global mean temperature records as the useful speedometer, which tells us that we have to step on the brakes in terms of emissions. 

This paper uses a new data set from the Caribbean to establish a long-term temperature record which appears to be well-correlated with global surface temperature trends. It shows that industrial-era warming began in the mid 1860’s, similar to previous research by Prof Nerilie Abram from the Australian National University and colleagues. It suggests that the total magnitude of warming since pre-industrial times (about 1.7°C) is much greater than the estimates built into the UNFCCC Paris Agreement (which indicate about 1.25°C warming). This implied different baseline temperature does not mean that we have to recast the 1.5°C and 2°C temperature goals but it does emphasise the duration and magnitude of human impact on global systems. It will be important to draw from other, similar, data sources beyond this one region to establish the global nature of these relationships.

New research shows that human-caused global warming started as early as the 1860s – several decades earlier than previously thought. The new research also shows with the correction for the early-onset global warming, we have already overshot the 2015 Paris Agreement threshold of 1.5° above pre-industrial global temperature – and are on track to exceed 2° of warming by the late 2020s. The study also shows the rate of global warming has increased in the past few decades.  

The new record of global temperature change comes from a type of sea sponge that grows a hard carbonate skeleton. The ratio in which the sea sponges store strontium versus calcium in their skeletons is directly related to the temperature of the water they grow in. The sea sponges used in this research grew in a part of the ocean that closely tracks global temperature.

For scientists to be able to calculate how much global temperature has risen above pre-industrial levels, we have to know exactly what the pre-industrial global temperature was. Temperature 'proxy' records like this, from natural archives, are extremely valuable because direct temperature measurements didn’t become widespread until well into the 1900s, when global warming had already started. This new record of global temperature from sea sponges starts in 1700 – long before the industrial era when humans started releasing large amounts of CO2 into the atmosphere. This means that the sea sponges are a 'natural thermometer' that provide an accurate pre-industrial temperature baseline.