Arctic News: wet bulb temperature

Showing posts with label wet bulb temperature. Show all posts

Friday, August 8, 2025

Extreme Heat Risk

High temperatures on land

The above image, adapted from NOAA National Weather Service, shows extreme heat risk for multiple areas in the U.S. for August 9, 2025, with a location near Imperial, California highlighted with details.
Details for the forecast for this location are:

- Wet bulb globe temperature: 93°F or 34°C
- Temperature: 102°F or 39°C
- Apparent Temperature: 117°F or 47°C
- Dew Point: 77°F or 25°C
- Relative Humidity: 46%
- Forecast for: August 9, 2025 21:00 UTC

On the above image, this extreme risk area is located at a latitude of 33.22° North. What is remarkable is that on the map there is also a high risk area that extends all the way from the southern border of the U.S. with Mexico to the northern border of the U.S. with Canada, with extreme risk areas showing up at latitudes North higher than for Toronto, Canada.

The image below, again adapted from NOAA National Weather Service, shows a wet bulb globe temperature forecast for August 11, 2025, with an extreme heat warning highlighted (inset) for a location near Imperial, California.

Details for the forecast for this location are:

- Wet bulb globe temperature: 95°F or 35°C
- Temperature: 109°F or 43°C
- Apparent Temperature: 121°F or 49°C
- Dew Point: 73°F or 23°C
- Relative Humidity: 32%
- Forecast for: August 11, 2025 21:00 UTC

[ from earlier post ]

The image below, adapted from the heat risk page at the NOAA National Weather Service, shows a forecast for August 9, 2025, updated 10.49 AM EST. The map shows high and extreme risk areas, including an extreme risk area centered around Grand Rapid, Michigan, which is located at a latitude of about 43° North.

The image below shows a heat stress forecast for August 11, 2025, with areas with extreme heat risk showing up in Michigan, while areas with major heat risk are showing up at latitudes as high as in Maine.

The image below, adapted from Climate Reanalyzer, shows a three-day forecast of maximum temperatures run on August 8, 2025.

The above images illustrate that extreme weather events that come with very high, even fatal heat stress conditions can now increasingly occur almost anywhere in the U.S.

Friederike Otto, climatologist at Imperial College London concludes: “Even relatively cold Scandinavian countries are facing dangerous heatwaves today – no country is safe from climate change".

High sea surface temperatures

In 2023, sea surface temperature anomalies first rose strongly (from 0.15°C on January 7, 2023, to 0.73°C on January 10, 2024). Then, sea surface temperature anomalies came down, in line with ENSO fluctuations (El Niño/La Niña). ENSO fluctuations and forecasts are also discussed in more detail further below.

Yet, over the past few months, sea surface temperature anomalies have been rising again, reaching an anomaly of 0.44°C from 1991-2020 on August 14, 2025, as illustrated by the image on the right, adapted from Copernicus and based on ERA5 data.

The image below, adapted from ClimateReanalyzer and based on NOAA OISST v2.1 data, shows sea surface temperatures through August 14, 2025. Sea surface temperatures have risen recently to very high levels, reaching 20.96°C on August 14, 2024, an anomaly from 1982-2010 of 0.72°C.

The image below, adapted from Climate Reanalyzer, shows the one-day average sea surface temperature anomaly (from 1971-2000) on August 7, 2025.

[ click on images to enlarge ]

Speeding up Arctic sea ice demise

Arctic sea ice declines due to rising ocean heat. The above image shows very high sea surface temperature anomalies around and inside the Arctic Ocean. These anomalies go up and down with the change in seasons, but they are getting higher over time due to rising Earth Energy Imbalance.

The image on the right, from an earlier post, illustrates the huge amounts of heat that have accumulated in the ocean, showing equivalent ocean heat content on August 9, 2025.

The image on the right underneath shows North Atlantic sea surface temperatures as high as 32.8°C on August 5, 2025. The image shows heat moving up along the path of the Gulf Stream toward the Arctic, threatening to cause more loss of sea ice and permafrost.

[ from earlier post, click to enlarge ]

Arctic sea ice also declines due to the sunlight heating up the sea ice. Where sea ice disappears, the water heats up rapidly. Arctic sea ice decline comes with feedbacks such as the albedo feedback, i.e. less sunlight getting reflected by sea ice means more heat is getting absorbed, further accelerating the temperature rise. More algae and soot settling on the sea ice can further contribute to albedo loss.

Feedbacks of the temperature rise can manifest as changes in heat sinks and buffers, with rapid impact on the temperature rise. Oceans constitute a huge buffer that has taken up huge amounts of heat. The capacity of oceans to take up heat threatens to diminish, e,g, due to stratification and changes in ocean currents, as discussed in earlier post such as this one.

Another buffer is the latent heat buffer that consumes heat in the process of melting snow and ice. Arctic sea ice is getting thinner over the years, and the amount of heat that can be absorbed in the process of melting is getting smaller over time. As the latent heat buffer diminishes, heat that was previously absorbed by the phase change from snow and ice to water, will therefore instead get absorbed by the water, further raising the temperature of the water. As sea ice thickness decreases over the years, less incoming ocean heat can be consumed by melting the remaining sea ice.

More freshwater temporarily slows down melting of Arctic sea ice

[ Bering Strait ]

Furthermore, Arctic sea ice decline is due to heat that is moving with the flow of rivers into the Arctic Ocean. The image on the right shows sea surface temperatures as high as 20.3°C in the Bering Strait on August 7, 2025.

Extreme weather events are getting more severe and are occurring more frequently, including heatwaves and thunderstorms on land that can extend over the Arctic Ocean. Rain falling on sea ice can speed up its demise. Heatwaves and storms over land can furthermore heat up the water of rivers and increase their flow, thus increasing the heat flowing into the Arctic Ocean.

Also, more evaporation of sea water takes place over the North Atlantic, with more precipitation falling further down the track of the Gulf Stream and its extension north. This also adds more freshwater in the Arctic.

Water from melting sea ice, from rivers and from precipitation is all freshwater, i.e. it contains no salt. The increase in freshwater at the surface of the Arctic Ocean has resulted in a temporary slowdown in the retreat of Arctic sea ice extent, due to a buffer that spans a maximum of 2°C (as depicted by image below on the right).

Freshwater buffer looks set to be overwhelmed soon

[ Saltier water, less sea ice, from earlier post ]

The higher the water's salt content, the lower its melting point. Seawater typically has a salinity of about 3.5% (35 grams of salt per liter of water).

Sea ice starts melting when the temperature rises to about -2°C (28.4°F). By contrast, freshwater remains frozen as long as the temperature remains below 0°C (32°F).

As said, the increase in freshwater at the surface of the Arctic Ocean slows down the retreat of Arctic sea ice extent, but this is only a temporary slowdown. Given the speed at which the temperature of the water of the Arctic Ocean keeps rising, this temporary slowdown looks set to be overwhelmed soon and rapid melting of sea ice looks set to return with a vengeance.

The above image shows Arctic sea ice concentration on August 14, 2025.

Arctic and Antarctic - two different situations

The image below, by Eliot Jacobson, illustrates the rise of precipitable water (total column) over the years.

Over the past two months (June-July 2025), the temperature over the Arctic Ocean has been slightly lower than 1951-1980, as illustrated by the image below. By contrast, areas with very high anomalies are visible between 60°S and 90°S. What's happening?

The image below shows that the precipitable water anomaly can be very high at both the North Pole and the South Pole. The image depicts the situation on August 9, 2025 18Z.

In the Northern Hemisphere, water evaporates from the sea surface of the North Atlantic and the North Pacific. Prevailing winds carry much water vapor in the direction of the Arctic. Precipitation over the Arctic Ocean freshens the surface, forming a buffer that temporarily slows down the decline of the sea ice extent. Similarly, much of the precipitation over land is carried by rivers into the Arctic Ocean, also freshening the surface of the Arctic Ocean. And of course, heavy melting of Arctic sea ice in June and July 2025 has added further freshwater to the surface of the Arctic Ocean.

The slowdown of AMOC can also create a buffer by delaying the transport of ocean heat toward the Arctic Ocean, but given the increase of Earth's Energy Imbalance and the additional heat that is instead accumulating in the north Pacific and the North Atlantic, more heat looks set to eventually reach the Arctic Ocean, overwhelming such buffers.

[ Precipitable water anomalies over Antarctica ]

In the Southern Hemisphere, water evaporates from the Southern Ocean and part of it falls on the Antarctic ice sheet, thickening the snow layer, as illustrated by the image on the right, showing a forecast of high precipitable water anomalies over Antarctica on August 20, 2025.

As a result, the Southern Ocean surface is getting more salty. As discussed in an earlier post, saltier surface waters sink more readily, allowing heat from the deep to rise, which can melt Antarctic sea ice from below, even during winter, making it harder for ice to reform. This vertical circulation also draws up more salt from deeper layers, reinforcing the cycle.

In conclusion, geographic differences result in different precipitation outcomes and this in turn causes salinity differences that are behind these temperature anomaly differences.

As said, the slowdown in the decline of Arctic sea ice extent that results from the increase in freshwater is temporary. Given the speed at which the temperature of the water of the Arctic Ocean keeps rising, this temporary slowdown looks set to be overwhelmed soon and rapid melting of sea ice looks set to return with a vengeance.

By contrast, the dramatic decrease in sea ice around Antarctica looks set to continue long-term, as a feedback that is amplified by albedo loss, lower emissivity, loss of the sea ice's latent heat buffer, ocean current changes and salinity changes.

Dire state of sea ice

The net result is illustrated by the image below. The global sea ice area anomaly was 2.62 million km² below the 1981-2010 mean on August 13, 2025, a standard deviation of -4.13σ from 1981-2010. The image shows that the global sea ice area anomaly was well below 1981-2010 in the years 2023, 2024 and 2025, which is remarkable, since there was a La Niña early in 2025. The year 2016 is also marked, since 2016 was a strong El Niño year.

The image below shows Arctic sea ice volume through August 11, 2025, when volume was at a record daily low, as it has been for more than a year.

High temperatures and dire state of sea ice despite borderline La Niña

What makes these high temperatures on land and the dire state of the sea ice even more significant is that there currently are no El Niño conditions. As illustrated by the image on the right, adapted from NOAA, the ENSO outlook (CFSv2 ensemble mean, black dashed line) favors borderline La Niña conditions during the Northern Hemisphere fall and early winter 2025-2026, which suppresses temperatures.

Over the past few months, there's been a zigzag pattern of rises and falls in sea surface temperatures in Niño 3.4, an area in the Pacific (inset) that is critical to the development of El Niño, as illustrated by the image below.

On August 11, 2025, the temperature in Niño 3.4 reached 26.51°C, an anomaly of 0.36°C vs 1991-2020. An El Niño event is defined by NOAA as an episode of at least five consecutive 3-month running mean sea surface temperature anomalies vs 1971-2000 surpassing the threshold of 0.5°C in the Niño 3.4 area, as illustrated by the image below.

The image on the right, adapted from ECMWF, shows the El Niño forecast through August 2026. The next El Niño may emerge soon, and it may continue to grow in strength in the course of 2026.

The temperature rise is accelerating and the rise could accelerate even more due to decreases in buffers (as described above), due to strengthening feedbacks, especially during an El Niño, and due to further reduction of the aerosol masking effect, which are all developments that could rapidly speed up existing feedbacks and trigger new feedbacks.

Seafloor methane

One of the most dangerous feedbacks is methane erupting from the seafloor of the Arctic Ocean. The image below shows hourly methane average recorded at the Barrow Atmospheric Baseline Observatory (BRW), a NOAA facility located near Utqiaġvik (formerly Barrow), Alaska, at 71.32 degrees North.

Climate Emergency Declaration

The situation is dire and the precautionary principle calls for rapid, comprehensive and effective action to reduce the damage and to improve the situation, as described in this 2022 post, where needed in combination with a Climate Emergency Declaration, as discussed at this group.