Global Surface Air Temperature (GloSAT)

Department Name: Science and Technology


Surface temperature is the longest instrumental record of climate change and the measure used in the Paris Climate Agreement that aims to 'prevent dangerous anthropogenic interference with the climate system'. The Agreement defines an ambition to limit global temperature change to 1.5C or 2C above pre-industrial levels. The Intergovernmental Panel on Climate Change (IPCC) used a baseline of 1850-1900 for its definition of 'pre-industrial' as this is when existing instrumental records begin. It has been estimated that global temperatures may have already increased by 0.0-0.2C by this time, but this is uncertain due to lack of data. However, even using the 1850-1900 baseline, existing temperature datasets disagree on the amount of warming to date and this disagreement implies more than 20% uncertainty in the allowed carbon budget to meet the goals of the Paris Agreement solely due to uncertainty in observed surface temperature change. These differences between temperature datasets arise mostly from two structural uncertainties: the use of sea surface temperatures (SST) rather than air temperatures over the oceans, especially ice-covered regions, and differences in data coverage and interpolation strategies. This project addresses both.

To best inform decision-makers, records of temperature change must be as accurate, consistent, and long as possible. Existing global datasets start in 1850 or later, but we will extend the record a further 70 years back to the late 18th century. Current knowledge of this period comes from instrumental measurements in Europe, palaeo-proxies (tree-rings, corals or ice cores), and climate models. We will dramatically extend the spatial coverage of the early measured record in this 70-year period, which is important for understanding natural climate variability and the climate response to different radiative forcings. For example, the longer record includes the period of 5 large volcanic eruptions and extra cycles of multi-decadal climate oscillations. The new record will allow us to better disentangle the contributions of anthropogenic and natural factors on the climate system and quantify the effect humans have already had on Earth's temperature, and hence on future climate.

A major inconsistency has been past use of air temperature over land but SST over oceans. Recent advances mean we can produce a marine air temperature record to construct the first global air temperature dataset over ocean, land and ice, stretching back to the late 18th century. Our dataset will be independent from SST, currently the most uncertain component of global temperature. We will improve land, marine and cryosphere air temperature observations to make them more homogeneous and extend the global record further back in time. This requires fundamental research to better understand the bias and noise characteristics of historical observations and develop new error models. We will adopt sophisticated statistical techniques to allow the estimation of air temperature everywhere, even when there are gaps in the observations. We will expand the historical climate record with new ship's logbook and weather station digitisations focused on early data, sparse periods and regions, and the interfaces between land, ocean and ice. We will engage the public in the digitisation effort building on recent successful citizen science initiatives.

We will analyse the new surface air temperature record to better understand how temperatures have changed since the late 18th century. This longer record will give a better understanding of natural climate variations, both variability generated internally within the climate system and that due to external forcing factors such as volcanic eruptions and solar changes. This improved understanding of natural variability will enable us to more cleanly isolate the characteristic "fingerprints" of man-made climate change allowing us to more confidently detect and attribute human-induced changes

Planned Impact

The most recent Climate Change Public Attitude Tracking Survey in the UK found that
74% of respondents were either 'very' or 'fairly' concerned about climate change. Global political concern over the issue is reflected by the Paris Agreement negotiated by 196 parties at the 21st Conference of Parties in 2015. The UK has committed to reducing greenhouse emissions by 80% by 2050. Clean growth is a key element of the government's industrial strategy.

The most visible measure of climate change for both policymakers and the public is the historical temperature record, which measures how global temperatures have changed over the past 170 years, and shows periods of rapid warming in the early 20th century and for the whole of the last 50 years. However the record is not long enough to give a complete picture of temperature change since before the start of the industrial revolution, or how exceptional recent temperature change is in comparison to pre-industrial conditions. The current historical temperature record mixes air and water temperatures in a way which creates confusion when comparing with climate model predictions.

This project will produce a new global surface air temperature dataset starting in around 1780 - at least 70 years longer than any other and stretching back almost to the start of the industrial revolution. The length of the record, the use of air temperatures for both land and oceans, and improvements to our understanding of the historical measurements will make this new dataset the benchmark for the understanding human impact on the climate for both policymakers and the general public.

Temperature data underpins national and international policy on climate change. The UN Framework Convention on Climate Change (UNFCCC) process will benefit from improved quantification of temperature change since the pre-industrial period to feed into their regular 'global stocktake' which will monitor progress towards achieving the aims of the Paris Agreement to limit global temperature change to 1.5 or 2C above pre-industrial levels.

A range of datasets with differing spatial and temporal resolutions will be produced to represent improving data coverage over time, and incorporating new analyses of the level of confidence in the data at any point in time. This information will feed into national & international climate assessments, such as the annual State of the Climate Report, the UK Climate Change Risk Assessments, and future Intergovernmental Panel on Climate Change reports. The climate science community will also benefit from an improved understanding of natural climate variations, such as those due to volcanic eruptions and internal ocean-atmosphere interactions: the new dataset will be widely used to compare to other observational datasets, weather model reanalyses and climate model simulations. This will improve our understanding of past climate change and weather extremes, and implications for future change, providing data for risk management in both government and industry.

An important part of GloSAT will be the digitisation of historical observations from their current paper or scanned image formats. Much of this will be achieved through public engagement and citizen science digitisation, building on the highly successful project, which has already rescued more than 2.5 million weather observations using thousands of volunteers. The rescued observations will be added to international weather observation databases such as ICOADS, ISTI, ISPD and the Copernicus Climate Data Store for the entire climate community to use. The use of citizen science for digitisation will also be exploited as an opportunity to engage the public in science and to communicate climate science.


10 25 50

publication icon
Brönnimann S (2024) Early 20th century Southern Hemisphere cooling in Climate of the Past

Title KCowtan/glosat-homogenisation: Code for preliminary analysis 
Description Initial release of code for GloSAT preliminary analysis. This release is missing estimation of uncertainties for station segment norms. 
Type Of Technology Software 
Year Produced 2022 
Open Source License? Yes  
Impact This code is supporting the preliminary GloSAT historical temperature analysis, which will be reported as a measure of historical global warming.