The State of the Climate report, published each year as a supplement to the Bulletin of the American Meteorological Society, is the most comprehensive annual global climate report, with contributions from hundreds of authors worldwide. The newly released State of the Climate in 2024 details record-breaking global greenhouse gas concentrations and surface temperatures as well as other key trends, events, and phenomena.
In this post, we speak with Global Climate chapter co-editors Kate Willett, Robert Dunn and Josh Blannin at the UK Met Office for their take on the report and significant events of 2024. Read the Global Climate chapter here [PDF].
What were your roles in the report?
At the Met Office (UK), we’ve been involved with the BAMS State of the Climate reports as editors and section authors since the early 2000s. Kate led the Global Climate chapter from the 2009 to 2015 reports and then handed over the baton to Robert. Josh joined as co-editor for the 2023 report. It’s been a real team effort with Kate managing the atmospheric hydrological cycle elements, Josh managing the temperature sections, Nadine Gobron (JRC-ISPRA) managing the cryosphere and terrestrial sections and Gary Morris (NOAA) managing the atmospheric composition sections. Robert manages the energy balance and circulation sections, the overview, many of the figures, and keeps us all in order and on time — a mammoth task!
In our other roles we’re busy in the weeds of data quality, dataset development and intercomparisons, amongst a myriad other things, so the annual report is a fantastic opportunity to step back and take stock of what is actually going on around us. In all honesty, over the last decade it has become an annual cold-water-over the head moment seeing the relentless tumble of records in so many climate variables year after year.
From the deep ocean to stratosphere, from oak leaf onset to permafrost thickness, everything is telling the same story — that emissions continue to increase; that the world is warming, unabated; and that there is deep complexity of the Earth-system response to that warming.
What are some noteworthy global trends or events you’d want to highlight from 2024?
Robert: Outside of the BAMS SotC my own work focuses on climate extremes, and so although near-surface high-temperature extremes metrics over land show increases since the 1980s, it is the fact that the fraction of the land surface setting new record values in a given year has been rising in recent years is for me personally interesting and a cause for concern. What I like about the report is the breadth of the metrics covered and some of the most interesting parts of the report for me are in sections far from my own work. For example, surface albedo is showing steady darkening from decreased snow and ice cover, and plant growth is showing steady increase, and is hence also affecting the albedo. Separately, the significant wildfire events and conditions during 2024, with Canada having another severe season (the second highest fire activity on record), but also South America which had the worst season since 2010.
Given the rapid changes that have been seen across many of the climate metrics in the chapter, it is hard to say what is “normal” at the current time. In Chapter 1 we show a figure of 36 panels depicting time series for metrics drawn from across the report. The vast majority of these show consistent, and in some cases rapid, changes in recent years/decades. Across both mine and Kate’s tenure as lead editor, some quantities have rarely failed to set new yearly records — e.g. glacier mass balance, sea level rise, and ocean heat content.
Kate: My own area of expertise is around water vapour and humidity, so the big story for me this year was the record-breaking near-surface specific humidity, frequency of high humid-heat days, and total column water vapour levels, for a second year in a row. Extreme precipitation over land was also record high.
I’ve been working with humidity observations for my entire career. My PhD was to develop a global humidity monitoring product and assess whether global humidity showed evidence of climate change. Back in 2003, my dataset showed that specific humidity (the amount of water vapour in the atmosphere) was increasing but that relative humidity (the saturation level) had remained fairly constant since the 1970s. My HadISDH product has now been a regular feature of the BAMS SotC since the 2010 report. At that point, the year with highest specific humidity anomaly over land (difference from normal) in the record (which begins in 1973) was 1998 at 0.26 g kg-1. The 2024 record reached 0.49 g kg-1, almost double the 1998 record. Over oceans, the 2024 record anomaly reached 0.56 g kg-1, almost tripling the 1998 record at 0.21 g kg-1. These are huge increases. Now, in 2024, with an extra 21 years of data compared to my original findings, we see that contrary to my original conclusions, relative humidity over land has decreased, and this is especially clear over the last two decades.
In essence, the amount of water vapour in the atmosphere has increased, but it has not increased as fast as it could, given the rise in temperature, so the relative humidity of the air (the saturation level) has decreased. The increase in water vapour isn’t keeping pace with the rise in temperature. This is largely because most of the water vapour is coming from over the oceans — our biggest water source. However, the oceans (and the air above them) have warmed slower than the air temperature over land. So, as air is transported around the globe, it doesn’t contain enough water vapour to keep pace with the rising temperatures over land.
How is our ability to monitor the climate evolving?
A number of new monitoring capabilities are presented in this year’s report. One is on lightning activity, where the new Lightning Imager on the Meteosat Third Generation satellite from EUMETSAT came online. Over time this will allow lightning to be monitored over Europe, Africa and parts of South America. Given the impact of lightning strikes on, for example, wildfire ignition and human infrastructure, this new platform will over time help in our understanding of changes linked to climate change.
Another is land surface temperature (LST), also measured from satellites. This quantity is the skin temperature of the Earth’s surface as viewed from the satellite, which can include a range of surface types including bare ground, urban areas, and forest canopies. LST products can provide information in areas without in situ temperature measurements and can be used to identify locations with exceptionally high temperatures. As more satellite LST datasets begin to reach lengths which are needed for climate studies, they are likely to be useful sources of independent temperature measurements.
Josh: Having records of the most significant events is really useful for confirming why we see extremes or distinct changes in our datasets. In my other work, I primarily use LST data and co-authored the [State of the Climate] sidebar on super extreme land surface temperature hotspots, where we identified regions of the Earth’s surface which exceeded 50° and 55°C during 2024. This showcases the potential of satellite-based LST for informative climate monitoring, and it was great to have this well received by the reviewers, and hopefully readers!
Image: Map (a) showing where two or more confirmed satellite observations in 2024 exceeded selected land surface temperature (LST) thresholds (25°, 30°, 35°, 40°, 45°, 50°, and 55°C), plus graphics showing how many 0.1° latitude–longitude grid sections of the globe exceeded 50°, and 55°C each year since 2019 (b) and the the annual fraction of valid observations in each year that have exceeded these thresholds across all grid sections (c). (Fig. SB2.3 from the “Global Climate” chapter in The State of the Climate in 2024; see report for full caption and explanation.)
The other sidebar in the Global Climates chapter this year also employs satellites, but for monitoring indirect short-lived climate forcers (SLCFs). SLCFs that provide a forcing element to the climate system themselves are considered direct SLCFs, whilst indirect SLCFs are precursors to these — the authors give the example of carbon monoxide and nitrogen oxides being ozone precursors. It’s an extremely informative piece on monitoring these indirect SLCFs, highlighting regions where we see the most emissions. It also nicely links to other parts of the report, such as the atmospheric composition section and biomass burning, demonstrating that many of these variables are closely linked.
What is it like to work on the report? Why are international scientific collaborations like this so important?
Robert: My role is both lead editor for the Global Climate chapter and also author of some of the sections. I produce many of the figures used in the chapter, from data supplied by our wonderful author teams. Although this is extra effort (and at times a little more stressful), it’s something that Kate started when she led the chapter, and I think helps the consistent look-and-feel and clarity of the chapter. It’s a real privilege to play a part in this long-running report series, and support the authors in presenting their updates.
By the time the report is published, many of the “standard” climate metrics have already been covered in other reports in great detail (e.g., surface temperature, greenhouse gas concentrations, etc). However, the BAMS SotC allows us to present a much greater variety of metrics and measures.
The BAMS SotC is a truly international team effort. Overall, the report has contributions from around 600 scientists across 58 countries, and Chapter 2 alone has over 250 authors. The report would be much smaller — in scope, size, and detail — without the time and effort from experts and contributors across the world. Already during the first half of 2025 we have seen some noteworthy events — from rockfalls in the Swiss Alps, to severe flash flooding, to record breaking wildfires in Europe and the United States. At a time when it is even more important to monitor our climate and its extreme events, to inform our understanding of its risks to our global society, we need to continue to work together as a global community to observe, monitor, report and archive information about the state of our climate.
Kate: I love the annual consistency of the BAMS SotC, which enables us to regularly status-check so many aspects of our Earth system. I also love the breadth of inclusion of different observation products from different platforms. The report includes the majority of available estimates of each variable from in situ, satellite, reanalysis, and any other relevant observing systems. This provides some insight into how confident we are in the trends and variability of that particular variable. For example, there is good agreement between the HadISDH in situ-based product (from ships and weather stations) and various reanalyses for specific humidity. This is less good for relative humidity, especially over the oceans, where confidence in relative humidity is low for both reanalyses and HadISDH.
Josh: I am relatively new to the BAMS State of the Climate process and began my involvement three years ago, starting as an internal reviewer, then becoming a co-editor managing the temperature section, and this year co-authoring one of the sidebars, which are used to introduce new variables into the report or describe a key climatic event from that year.
The report takes many people many hours to write, edit and compile, but the result is well worth it, with the final product providing a broad and comprehensive, yet accessible, summary of the global climate. Working with authors and editors across the world to report on climate variables which influence the Earth is a great reminder of how solving climate change will require a collaborative effort. In fact, I would say the State of the Climate is quite symbolic of how we can achieve so much more when working together.
Image at Top: Montage of climate data visualizations. Clockwise from top left: 1) ERA5 surface specific humidity anomalies (g kg−1). 2) Global Rx1day anomalies in 2024 with respect to the 1991–2020 mean from Multi-Source Weighted-Ensemble Precipitation (MSWEP; Beck et al. 2019) highlighting a band of wet anomalies across the tropics. 3) HadISDH.extremes humid heat frequency anomalies (TwX90p), measured by the number of days where the daily maximum wet-bulb temperature exceeds the local daily 90th percentile (days yr−1). Gray background (over land) represent regions with insufficient data. 4) HadCRUT5 surface air temperature anomalies, 1991–2010.
