The Earth’s jet streams play a fundamental role in the speed and direction of weather systems across the world.
This means that they are crucial for understanding extreme weather events and how they will change as the world warms.
Research suggests that upper-level jet-stream winds will accelerate on average as global temperatures rise, but little is known about how their fastest winds – known as “jet streaks” – will change.
In a first-ever study, published in Nature Climate Change, my co-author and I show that fast jet-stream winds will get faster and faster — by around 2% for every degree Celsius the world warms. This means that fast winds will speed up around 2.5 times more than average jet-stream winds.
Furthermore, it means we should anticipate record-breaking jet-stream winds as warming continues.
Our research also reveals that this acceleration occurs because the difference between the density of the air in the tropics and the air at the poles will increase.
While further work will be needed to understand the full impact of our findings, we expect that they will include stronger severe storms and an increase in clear-air turbulence for aircraft passengers.
Fast flowing
The Earth’s jet streams are fast-flowing narrow bands of wind high up in the atmosphere. The fastest jet-stream winds blow from west to east and occur in the upper troposphere, around 10-12 km above the surface.
Jet streams are important because they shape Earth’s surface climate by steering weather systems, and so they can affect where severe weather occurs. For example, the regions around fast upper-level jet-stream winds – called “jet streaks” – have been linked to the occurrence of storms, tornadoes, hail and severe winds.
Jet streams are also key for air travel, providing an ideal tailwind for aircraft. Previous research has established that the average wind speed of the upper-level jet stream increases under climate change. This has the potential knock-on effect of causing more clear-air turbulence for aircraft passengers.
Our research was inspired by reports in 2019 of transatlantic flights breaking speed records. As a result, we set out to find out how climate change will affect fast jet-stream winds.
Little is known about how fast upper-level jet-stream winds – classed as those above the 99th percentile – could change as the world warms. Furthermore, no mechanism has been proposed to explain why fast jet-stream winds would change.
Fast-get-faster response
We started by examining how physics-based climate models project fast jet-stream winds would change. We used models from the sixth Coupled Model Intercomparison Project (CMIP6), which were developed for the latest assessment by the Intergovernmental Panel on Climate Change (IPCC).
In these model projections, we compare daily jet-stream winds that exceed the 99th percentile at the end of the 20th (1980-2000) and 21st (2080-2100) centuries under a very high emissions scenario (SSP5-8.5). We also compare wind speeds for the near-term in an intermediate scenario (SSP2-4.5), which is broadly in line with the trajectory of global emissions today.
Our analysis finds that climate change makes the fastest upper-level jet-stream winds get faster and faster – by about 2% for every degree Celsius the world warms. This means that fast winds will speed up around 2.5 times more than average jet-stream winds.
We refer to this as the “fast-get-faster” response and we find the effect in all seasons of the year.
You can see this in the chart below, which shows the percentage change in the fastest winds at around 12km altitude per degree of warming across different latitudes (from 80 degrees south on the left-hand side to 80 degrees north on the right).
While the fast winds increase at all latitudes, those in the “extratropics” – that is, between around 20 to 60 degrees, where jet streams are found – are the fastest to begin with and thus get the largest boost under climate change.
Percentage changes in fast (>99th percentile) winds at 200 hectopascal (hPa), normalised by the global average change in surface air temperature for each climate model from 80 degrees south to 80 degrees north in latitude. Simulations use SSP5-8.5. The black line indicates the multi-model average and the shading indicates one standard deviation of the response across all the models. Source: Shaw & Miyawaki (2023)
Moist air
In addition to quantifying the “signal” of long-term change, we also provide a physical explanation for why it occurs.
Bridging the gap between simulating the response to climate change using models and understanding the causes helps us justify that this is a signal to take seriously.
The first step we take is to simplify the model to help isolate what physics underlies the signal. When the model is run without ocean currents and without land, we still find the signal.
This suggests that the fast-get-faster signal emerges in a world formed entirely of water. The result implies that the physics of a moist atmosphere is key to explaining the fast-get-faster response.
The second step we take is to use our physical understanding of the jet stream to quantify the connection between moisture and the signal.
The jet stream exists because of the contrast of density between air at the equator, which is warm and light, and air at the pole, which is cold and dense. We connect this contrast to the response of moisture under climate change.
In particular, in today’s climate, tropical air holds more moisture than air at the poles because it is warmer. Climate change exacerbates this contrast because hotter air can hold much more moisture than colder air.
While the air at the poles is warming more rapidly than in the tropics, hotter air can hold so much more moisture than cold air that the overall density difference still increases.
This effect increases the density contrast under climate change, accelerating the jet-stream winds. Importantly, the effect is multiplicative – namely, fast jet-stream winds today that involve a steep density contrast would be boosted much more in the future than slower jet-stream winds that involve a shallower density contrast.
Thus, our results project record-breaking jet-stream winds.
Emerging signal
When we look at the recent past (1980–2022) using reanalysis data – which combines physical observations with model simulations – we do not find that the fast-get-faster signal has yet emerged from the noise of natural variability.
However, all the climate models in our study suggest that a statistically significant fast-get-faster signal will emerge for the extratropics in both the southern and northern hemispheres by the middle of this century.
Specifically, under SSP2-4.5, all climate models project the signal in the southern and northern hemispheres extratropics by 2038 and 2048, respectively. Under SSP5-8.5, this is slightly earlier – by 2035 and 2045, respectively.
This is shown in the figures below, which show the percentage change in fast jet-stream winds, relative to 1980-2000, from 1980 to 2050 in the southern (top) and northern (bottom) hemispheres, excluding the tropics. The lines indicate reanalysis data (black) and climate models projections under SSP2-4.5 (green) and SSP5-8.5 (orange).
The charts on the right-hand side show the trend, per degree of warming, for each model (green and orange) and the reanalysis data (black). Closed and open circles indicate results that are and are not statistically significant, respectively.
Timeseries of percentage changes (relative to 1980-2000) in fast 200hPa jet-stream winds in reanalysis and climate models for different emission scenarios for the southern (top) and northern (bottom) hemisphere extratropics from 1980 to 2050. Data are presented as multi-model average (thick line) with one standard deviation of the response across the models (shading). Right-hand charts show the linear trends of these changes per degree of global warming, where statistically significant trends are indicated by closed circles. Source: Shaw & Miyawaki (2023)
We are now working to better understand the knock-on impacts of these changes in the jet stream for severe weather.
New climate models are allowing scientists to look in greater detail at how extreme weather is – and will – change. Ultimately, unravelling the impacts of climate change on winds at regional scales will help society better prepare for the implications of a warming world.
The post Guest post: Why ‘jet-streak’ winds will get faster as the climate warms appeared first on Carbon Brief.
Guest post: Why ‘jet-streak’ winds will get faster as the climate warms
Vishal Prasad is director of Pacific Islands Students Fighting Climate Change.
When the International Court of Justice (ICJ) delivered its advisory opinion on climate change last year, it marked a turning point not just for the Pacific, but for international climate law.
The court was unambiguous: states have legal obligations to protect the environment from greenhouse gas emissions, and they face accountability when they fail. For those of us who carried this campaign from a classroom in Vanuatu to Europe and New York, it was a moment of profound validation.
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But we have always said that the advisory opinion was a tool, not an endpoint. The ICJ affirmed what many in the Pacific have been saying for some time. Now we have a legal blueprint, we must carry this momentum from the courtrooms to the negotiating rooms.
Potential to shape climate politics
The advisory opinion has already begun to reshape the climate landscape. At COP30 in Belém, we saw countries that had supported the campaign citing the opinion in their interventions, while those blocking progress were clearly concerned of its implications. Its potential to shape climate politics and policy is significant.
This year we have arrived at the mid-year climate negotiations in Bonn not only with the advisory opinion, but with a UN General Assembly resolution endorsing it. Despite a fierce campaign from the usual suspects, just eight countries, including the USA, Saudi Arabia, Russia and Iran voted against. That is a victory for multilateralism at a moment when multilateralism is under strain.
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But we know that advisory opinions alone are not enough. Legal clarity will not automatically translate into reduced emissions, increased finance flows or stronger national climate plans. That translation requires political will in the negotiating rooms, both here in Bonn and all the way through Fiji and finally in Antalya this November.
What the Pacific needs from this negotiating year
The Pacific put significant political capital into the joint Australia-Pacific bid for COP31. It is fair to say that the compromise of Australia holding the role of president of negotiations while the COP is held and presided over by Türkiye is not what we imagined.
But we in the Pacific are used to looking for silver linings. Both Australia and Türkiye have acknowledged the important role the Pacific will have at COP31, through the appointment of Pacific champions and the hosting of a Pacific Pre-COP in Fiji with a leaders event in Tuvalu. These are genuine opportunities to bring the world to our shores and ensure that Pacific issues are front and centre going into the final negotiations.
But we are not naive. Envoy positions and meeting locations are just the architecture of goodwill. We need to see that goodwill converted into concrete negotiating outcomes and finance.
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The Pacific helped put Australia’s climate minister Chris Bowen in this important position, so we expect to see Australia advocate not only for us, but to turn a mirror towards itself as one of the world’s biggest fossil fuel exporters.
At Bonn, and then in Antalya, we need ambition on mitigation that reflects the ICJ’s clarity on state obligations and the science. That means action on fossil fuels.
We need climate finance that is new, additional and accessible to the countries that need it most. In the Pacific we have already demonstrated what that looks like.
The Pacific Resilience Facility is the first climate finance facility designed, governed and managed by Pacific people, built specifically to reach the grassroots and community initiatives that larger funds routinely bypass. We need the international community to meet that ambition with contributions that reflect climate justice, starting with pledges to meet the $500-million capitalisation goal.
And we need the oceans – which are the lifeblood of the Pacific and a critical part of the global climate system – treated as a central element of the negotiations rather than a thematic aside.
Energy crisis driven by imported fossil fuels
The days of speaking about climate and fossil fuels purely as a moral issue are long gone. Pacific ministers recently adopted the Tassiriki Call for a Fossil Fuel Free Pacific, in the context of a deepening energy crisis that has triggered states of emergency in several Pacific nations. Our dependence on imported fossil fuels is both a climate and an economic vulnerability.
Conflict in the Middle East is pushing our region into an energy crisis. We are dependent on imported fossil fuels for 80% of our energy needs. My home country of Fiji could see an increased fuel bill of nearly three times our annual healthcare budget.
Comment: COP31 must persuade countries to make fossil fuel transition plans
We need the technical and financial support to transition to 100% renewable energy. Not only because it is what the world owes us for decades of carbon pollution that continue to render parts of our home uninhabitable, damaging ecosystems and culture. But because we must be part of that transition. Fossil fuels have proven to be the greatest source of damage to our climate, and with their volatility, to our sovereignty as well.
What next?
The demands have not changed. Greater action on mitigation, adaptation, finance, loss and damage: these remain the substance of what the Pacific requires from the international community. What has changed is the legal foundation beneath them.
The ICJ has affirmed that these are not requests. They are obligations. The task this year is to make the negotiations reflect that.
The post The Pacific made history in the courts – now we must do it in the negotiations appeared first on Climate Home News.
The Pacific made history in the courts – now we must do it in the negotiations
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