New data raises questions about how much the Earth has warmed

Scientists are reevaluating historical warming trends thanks to a new temperature dataset that extends further back than previous records. This reexamination is prompting questions about just how much the Earth has warmed. For decades, climate models have used 1850 as a starting point for measuring global temperature changes. That said, the reality is a bit more complicated. the new dataset, called GloSAT, goes back to 1781, offering a more comprehensive view of pre-industrial temperatures.

Prior to the Industrial Revolution, which began with James Watt's steam engine in 1769, and even before the surge in planet-warming pollution after World War II, human activities like deforestation had already begun to release carbon into the atmosphere. Traditionally, climate scientists start measuring the planet's temperature from 1850 onward, but the new data suggests that this approach may not capture the full extent of early human-caused warming.

The extended timeframe matters because greenhouse gas concentrations rose by 2.5% between 1750 and 1850. This increase was enough to cause some warming that earlier data sets haven't accounted for. The GloSAT data adds to a growing scientific consensus that the Earth has warmed more than previously estimated based on the 1850 baseline.

According to Colin Morice, the lead author of the GloSAT study and a scientist at the UK's Met Office Hadley Centre, the 1850 start date was chosen primarily for practical reasons, based on data availability. He notes that industrialization did not begin in 1850.

The study, published in Earth System Science Data by a team of 16 scientists, reveals that the Earth was significantly cooler between the late 1700s and 1849 than during the period of 1850-1900. Scientists have defined the latter period as the "pre-industrial" baseline for assessing temperature change.

It's important to note that not all of the warming between these early periods can be attributed to human activities. Major volcanic eruptions in the early 1800s, like the 1815 eruption of Tambora, released particles into the stratosphere that blocked sunlight and caused a cooling effect. Ed Hawkins, a researcher with the National Centre for Atmospheric Science and a study author, points out that an 1808 eruption, nearly as large as Tambora, also contributed to this cooling, though its location remains unknown.

Some of the warming observed by the late 19th century likely represents a natural recovery from these volcanic events. That said, the reality is a bit more complicated. a 2021 report by the U.N. Intergovernmental Panel on Climate Change (IPCC) concluded that human activities likely caused some warming between 1750 and 1850, estimating it to be between 0 and 0.2 degrees Celsius. The GloSAT scientists' findings align with the middle of that range.

A related study, accepted in the journal Environmental Research Letters, used the GloSAT dataset and climate models to further investigate the human contribution to warming between 1750 and 1850. Led by Andrew Ballinger from the University of Edinburgh, the study attributes 0.09 degrees Celsius of warming to human activities, accounting for the diminishing effects of volcanic eruptions.

Andrew Schurer, a researcher at the University of Edinburgh and one of the study's authors, emphasizes the significance of this period due to the substantial volcanic activity. Piers Forster, a climate scientist at the University of Leeds, reached similar conclusions in a separate study last year, using the relationship between atmospheric carbon dioxide levels and planetary temperature. His research suggests that the early rise in carbon dioxide had a notable impact, estimating an additional 0.1 to 0.2 degrees Celsius of warming due to human activity before 1850.

Reconstructing the Past: Old Records, New Insights

While global temperature records weren't systematically kept centuries ago, numerous individuals documented local temperature changes. By combining these efforts, modern researchers can piece together a global picture. These early temperature records are intertwined with the history of science, exploration, and commercial activities.

Temperature measurements began in Europe in the 17th century and spread to North America and beyond. The Central England Temperature series, the longest of its kind, dates back to 1659. A temperature record from Uppsala, Sweden, began in 1722, thanks to the work of Anders Celsius.

These historical records are incorporated into the GloSAT analysis, revealing significant warming in some locations. For example, at Hohenpeissenberg, in the Bavarian Alps, Augustinian clerics have been recording temperatures since 1781. Wolfgang Steinbrecht, a scientist with the German Weather Service, notes that this record has survived remarkably intact, despite disruptions like the Napoleonic Wars. The Hohenpeissenberg record shows a regional warming of nearly 3 degrees Celsius when comparing the last 10 years with the period between 1781 and 1849.

That said, the reality is a bit more complicated. deducing the planet's overall temperature requires more than just land-based measurements. Oceans cover approximately 70% of the Earth's surface, making ocean temperature data crucial. Although not as systematic as land-based observations, ships did measure temperatures in the 18th century. The British East India Company, for example, recorded air pressure, temperature, and other data during their voyages between Europe, India, and China. Elizabeth Kent, a scientist with the National Oceanography Centre in the UK and a study author, explains that these measurements were taken for competitive advantage, helping them optimize trade routes.

Whaling vessels also recorded temperatures in the Atlantic Ocean, and these records were compiled by Matthew Fountaine Maury, a US naval officer and early oceanographer. The Atlantic and Indian Oceans were better measured than the Pacific due to established trade routes.

The GloSAT dataset utilizes these marine air temperature measurements, which were more commonly taken aboard ships during these early years. The authors acknowledge that data becomes sparser further back in time, resulting in greater uncertainty in temperature estimates for the earlier periods. Peter Thorne, a climate scientist at Maynooth University in Ireland and a reviewer of the study, confirms that while uncertainty is higher before 1850, the data undeniably indicates a cooler Earth.

Implications for Understanding Climate Change

If the Earth experienced additional warming before 1850, how does this change our understanding of the current state of the planet and the extent of human impact? While it might seem discouraging to learn that climate change is an even larger problem than previously thought, scientists caution against drawing overly simplistic conclusions.

Zeke Hausfather, a researcher with Berkeley Earth familiar with the GloSAT project, considers it a significant advancement. That said, the reality is a bit more complicated. he advises against assuming that this early warming undermines climate goals like the Paris Agreement, which are generally based on the 1850-1900 baseline. Hausfather believes the data provides valuable insights into pre-1850 warming but doesn't necessarily alter our ability to achieve climate targets.

Peter Thorne agrees that the finding of earlier warming isn't irrelevant but requires careful interpretation. He states that while it confirms we've warmed the Earth more than previously believed, the impacts are already occurring and have been calculated relative to more recent reference periods.

Accounting for this additional warming is crucial for accurately assessing the total human impact on the Earth and its potential influence on Earth systems. Ultimately, Thorne views this information as part of a larger picture that reinforces the urgency of addressing the potential for increasing climate impacts. He concludes that it shifts our perception of how far we have already pushed the climate system in important ways.