Earth science – Page 2

Model mixes AI and physics to do global forecasts

atmospheric science, climate, Earth science, forecasting, meteorology, Science, Weather / Mike M. / July 22, 2024

Cloudy with a chance of accuracy —

Google/academic project is great with weather, has some limits for climate.

John Timmer – Jul 22, 2024 5: 45 pm UTC

Image of a dark blue flattened projection of the Earth, with lighter blue areas showing the circulation of the atmosphere. — Enlarge / Image of some of the atmospheric circulation seen during NeuralGCM runs.

Google

Right now, the world’s best weather forecast model is a General Circulation Model, or GCM, put together by the European Center for Medium-Range Weather Forecasts. A GCM is in part based on code that calculates the physics of various atmospheric processes that we understand well. For a lot of the rest, GCMs rely on what’s termed “parameterization,” which attempts to use empirically determined relationships to approximate what’s going on with processes where we don’t fully understand the physics.

Lately, GCMs have faced some competition from machine-learning techniques, which train AI systems to recognize patterns in meteorological data and use those to predict the conditions that will result over the next few days. Their forecasts, however, tend to get a bit vague after more than a few days and can’t deal with the sort of long-term factors that need to be considered when GCMs are used to study climate change.

On Monday, a team from Google’s AI group and the European Centre for Medium-Range Weather Forecasts are announcing NeuralGCM, a system that mixes physics-based atmospheric circulation with AI parameterization of other meteorological influences. Neural GCM is computationally efficient and performs very well in weather forecast benchmarks. Strikingly, it can also produce reasonable-looking output for runs that cover decades, potentially allowing it to address some climate-relevant questions. While it can’t handle a lot of what we use climate models for, there are some obvious routes for potential improvements.

Meet NeuralGCM

NeuralGCM is a two-part system. There’s what the researchers term a “dynamical core,” which handles the physics of large-scale atmospheric convection and takes into account basic physics like gravity and thermodynamics. Everything else is handled by the AI portion. “It’s everything that’s not in the equations of fluid dynamics,” said Google’s Stephan Hoyer. “So that means clouds, rainfall, solar radiation, drag across the surface of the Earth—also all the residual terms in the equations that happen below the grid scale of about roughly 100 kilometers or so.” It’s what you might call a monolithic AI. Rather than training individual modules that handle a single process, such as cloud formation, the AI portion is trained to deal with everything at once.

Critically, the whole system is trained concurrently rather than training the AI separately from the physics core. Initially, performance evaluations and updates to the neural network were performed at six-hour intervals since the system isn’t very stable until at least partially trained. Over time, those are stretched out to five days.

The result is a system that’s competitive with the best available for forecasts running out to 10 days, often exceeding the competition depending on the precise measure used (in addition to weather forecasting benchmarks, the researchers looked at features like tropical cyclones, atmospheric rivers, and the Intertropical Convergence Zone). On the longer forecasts, it tended to produce features that were less blurry than those made by pure AI forecasters, even though it was operating at a lower resolution than they were. This lower resolution means larger grid squares—the surface of the Earth is divided up into individual squares for computational purposes—than most other models, which cuts down significantly on its computing requirements.

Despite its success with weather, there were a couple of major caveats. One is that NeuralGCM tended to underestimate extreme events occurring in the tropics. The second is that it doesn’t actually model precipitation; instead, it calculates the balance between evaporation and precipitation.

But it also comes with some specific advantages over some other short-term forecast models, key among them being that it isn’t actually limited to running over the short term. The researchers let it run for up to two years, and it successfully reproduced a reasonable-looking seasonal cycle, including large-scale features of the atmospheric circulation. Other long-duration runs show that it can produce appropriate counts of tropical cyclones, which go on to follow trajectories that reflect patterns seen in the real world.

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The Earth heated up when its day was 22 hours long

atmospheric science, climate models, Climatology, Earth science, geology, geoscience, Science / DJ Henderson / July 3, 2024

Because most things about Earth change so slowly, it’s difficult to imagine them being any different in the past. But Earth’s rotation has been slowing due to tidal interactions with the Moon, meaning that days were considerably shorter in the past. It’s easy to think that a 22-hour day wouldn’t be all that different, but that turns out not to be entirely true.

For example, some modeling has indicated that certain day lengths will be in resonance with other effects caused by the planet’s rotation, which can potentially offset the drag caused by the tides. Now, a new paper looks at how these resonances could affect the climate. The results suggest that it would shift rain to occurring in the morning and evening while leaving midday skies largely cloud-free. The resulting Earth would be considerably warmer.

On the Lamb

We’re all pretty familiar with the fact that the daytime Sun warms up the air. And those of us who remember high school chemistry will recall that a gas that is warmed will expand. So, it shouldn’t be a surprise to hear that the Earth’s atmosphere expands due to warming on its day side and contracts back again as it cools (these lag the daytime peak in sunlight). These differences provide something a bit like a handle that the gravitational pulls of the Sun and Moon can grab onto, exerting additional forces on the atmosphere. This complicated network of forces churns our atmosphere, helping shape the planet’s weather.

Two researchers, Russell Deitrick and Colin Goldblatt at Canada’s University of Victoria, were curious as to what would happen to these forces as the day length got shorter. Specifically, they were interested in a period where the day’s length would be at resonance with phenomena called Lamb waves.

Lamb waves aren’t specific to the atmosphere. Rather, they’re a specific manner in which a disturbance can travel through a medium, from vibrations in a solid to sound through the air.

Although various forces can create Lamb waves in the atmosphere, they’ll travel with a set of characteristic frequencies. One of those is roughly 10.5 to 11 hours. As you go back in time to shorter days, you’ll reach a point where the Earth’s day was a bit shorter than 22 hours, or twice the period of the Lamb waves. At this point, any disturbances in the atmosphere related to day length would have the ability to interact with the Lamb waves that were set off the day prior. This resonance could potentially strengthen the impact of any atmospheric phenomena related to day length.

Figuring out whether they do turned out to be a bit of a challenge. There are plenty of climate models to let researchers explore what’s going on in the modern atmosphere. But a lot of these have key features, like day length and solar output, hard coded into them. Others don’t let you do things like rearrange the Earth’s continents or change some atmospheric components.

The researchers did find a model that would allow them to change day length, solar intensity, and carbon dioxide concentrations to those present when Earth’s day length was 22 hours (which was likely to be in the pre-Cambrian). But it wasn’t able to reset the ozone concentrations, and ozone is also a greenhouse gas. So, they ran simulations without ozone, which are expected to be an under-estimate, and one where they elevated methane concentrations in order to mimic ozone’s greenhouse effect.

The Earth heated up when its day was 22 hours long Read More »

Is a colonial-era drop in CO₂ tied to regrowing forests?

carbon dioxide, climate change, colonization, Earth science, Ice cores, reforestation, Science / Mike M. / June 2, 2024

More trees, less carbon —

Carbon dioxide dropped after colonial contact wiped out Native Americans.

Howard Lee – Jun 2, 2024 11: 00 am UTC

Image of a transparent disk against a blue background. The disk has lots of air bubbles embedded in it. — Enlarge / A slice through an ice core showing bubbles of trapped air.

British Antarctic Survey

Did the massive scale of death in the Americas following colonial contact in the 1500s affect atmospheric CO₂ levels? That’s a question scientists have debated over the last 30 years, ever since they noticed a sharp drop in CO₂ around the year 1610 in air preserved in Antarctic ice.

That drop in atmospheric CO₂ levels is the only significant decline in recent millennia, and scientists suggested that it was caused by reforestation in the Americas, which resulted from their depopulation via pandemics unleashed by early European contact. It is so distinct that it was proposed as a candidate for the marker of the beginning of a new geological epoch—the “Anthropocene.”

But the record from that ice core, taken at Law Dome in East Antarctica, shows that CO₂ starts declining a bit late to match European contact, and it plummets over just 90 years, which is too drastic for feasible rates of vegetation regrowth. A different ice core, drilled in the West Antarctic, showed a more gradual decline starting earlier, but lacked the fine detail of the Law Dome ice.

Which one was right? Beyond the historical interest, it matters because it is a real-world, continent-scale test of reforestation’s effectiveness at removing CO₂ from the atmosphere.

In a recent study, Amy King of the British Antarctic Survey and colleagues set out to test if the Law Dome data is a true reflection of atmospheric CO₂ decline, using a new ice core drilled on the “Skytrain Ice Rise” in West Antarctica.

Precious tiny bubbles

In 2018, scientists and engineers from the British Antarctic Survey and the University of Cambridge drilled the ice core, a cylinder of ice 651 meters long by 10 centimeters in diameter (2,136 feet by 4 inches), from the surface down to the bedrock. The ice contains bubbles of air that got trapped as snow fell, forming tiny capsules of past atmospheres.

The project’s main aim was to investigate ice from the time about 125,000 years ago when the climate was about as warm as it is today. But King and colleagues realized that the younger portion of ice could shed light on the 1610 CO₂decline.

“Given the resolution of what we could obtain with Skytrain Ice Rise, we predicted that, if the drop was real in the atmosphere as in Law Dome, we should see the drop in Skytrain, too,” said Thomas Bauska of the British Antarctic Survey, a co-author of the new study.

The ice core was cut into 80-centimeter (31-inch) lengths, put into insulated boxes, and shipped to the UK, all the while held at -20°C (-4°F) to prevent it from melting and releasing its precious cargo of air from millennia ago. “That’s one thing that keeps us up at night, especially as gas people,” said Bauska.

In the UK they took a series of samples across 31 depth intervals spanning the period from 1454 to 1688 CE: “We went in and sliced and diced our ice core as much as we could,” said Bauska. They sent the samples, still refrigerated, off to Oregon State University where the CO₂ levels were measured.

The results didn’t show a sharp drop of CO₂—instead, they showed a gentler CO₂ decline of about 8 ppm over 157 years between 1516 and 1670 CE, matching the other West Antarctic ice core.

“We didn’t see the drop,” said Bauska, “so we had to say, OK, is our understanding of how smooth the records are accurate?”

A tent on the Antarctic ice where the core is cut into segments for shipping.

British Antarctic Survey

To test if the Skytrain ice record is too blurry to show a sharp 1610 drop, they analyzed the levels of methane in the ice. Because methane is much less soluble in water than CO₂, they were able to melt continuously along the ice core to liberate the methane and get a more detailed graph of its concentration than was possible for CO₂. If the atmospheric signal was blurred in Skytrain, it should have smoothed the methane record. But it didn’t.

“We didn’t see that really smoothed out methane record,” said Bauska, “which then told us the CO₂ record couldn’t have been that smoothed.”

In other words, the gentler Skytrain CO₂ signal is real, not an artifact.

Does this mean the sharp drop at 1610 in the Law Dome data is an artifact? It looks that way, but Bauska was cautious, saying, “the jury will still be out until we actually get either re-measurements of the Law Dome, or another ice core drilled with a similarly high accumulation.”

Is a colonial-era drop in CO₂ tied to regrowing forests? Read More »

East Coast has a giant offshore freshwater aquifer—how did it get there?

aquifers, Earth science, fresh water, geology, glaciers, Science / DJ Henderson / May 20, 2024

Image of a large boat with a tall tower at its center, and a crane in the rear. It is floating on a dark blue ocean and set in front of a white cloud. — Enlarge / An oceangoing scientific drilling vessel may be needed to figure out how huge undersea aquifers formed.

One-quarter of the world’s population is currently water-stressed, using up almost their entire fresh water supply each year. The UN predicts that by 2030, this will climb to two-thirds of the population.

Freshwater is perhaps the world’s most essential resource, but climate change is enhancing its scarcity. An unexpected source may have the potential to provide some relief: offshore aquifers, giant undersea bodies of rock or sediment that hold and transport freshwater. But researchers don’t know how the water gets there, a question that needs to be resolved if we want to understand how to manage the water stored in them.

For decades, scientists have known about an aquifer off the US East Coast. It stretches from Martha’s Vineyard to New Jersey and holds almost as much water as two Lake Ontarios. Research presented at the American Geophysical Union conference in December attempted to explain where the water came from—a key step in finding out where other undersea aquifers lie hidden around the world.

As we discover and study more of them, offshore aquifers might become an unlikely resource for drinking water. Learning the water’s source can tell us if these freshwater reserves rebuild slowly over time or are a one-time-only emergency supply.

Reconstructing history

When ice sheets sat along the East Coast and the sea level was significantly lower than it is today, the coastline was around 100 kilometers further out to sea. Over time, freshwater filled small pockets in the open, sandy ground. Then, 10,000 years ago, the planet warmed, and sea levels rose, trapping the freshwater in the giant Continental Shelf Aquifer. But how that water came to be on the continental shelf in the first place is a mystery.

New Mexico Institute of Mining and Technology paleo-hydrogeologist Mark Person has been studying the aquifer since 1991. In the past three decades, he said, scientists’ understanding of the aquifer’s size, volume, and age has massively expanded. But they haven’t yet nailed down the water’s source, which could reveal where other submerged aquifers are hiding—if we learn the conditions that filled this one, we could look for other locations that had similar conditions.

“We can’t reenact Earth history,” Person said. Without the ability to conduct controlled experiments, scientists often resort to modeling to determine how geological structures formed millions of years ago. “It’s sort of like forensic workers looking at a crime scene,” he said.

Person developed three two-dimensional models of the offshore aquifer using seismic data and sediment and water samples from boreholes drilled onshore. Two models involved ice sheets melting; one did not.

Then, to corroborate the models, Person turned to isotopes—atoms with the same number of protons but different numbers of neutrons. Water mostly contains Oxygen-16, a lighter form of oxygen with two fewer neutrons than Oxygen-18.

Throughout the last million years, a cycle of planetary warming and cooling occurred every 100,000 years. During warming, the lighter ¹⁶O in the oceans evaporated into the atmosphere at a higher rate than the heavier ¹⁸O. During cooling, that lighter oxygen came down as snow, forming ice sheets with lower levels of ¹⁸O and leaving behind oceans with higher levels of ¹⁸O.

To determine if ice sheets played a role in forming the Continental Shelf Aquifer, Person explained, you have to look for water that is depleted in ¹⁸O—a sure sign that it came from ice sheets melting at their base. Person’s team used existing global isotope records from the shells of deep-ocean-dwelling animals near the aquifer. (The shells contain carbonate, an ion that includes oxygen pulled from the water).

Person then incorporated methods developed by a Columbia graduate student in 2019 that involve using electromagnetic imaging to finely map undersea aquifers. Since saltwater is more electrically conductive than freshwater, the boundaries between the two kinds of water are clear when electromagnetic pulses are sent through the seafloor: saltwater conducts the signal well, and freshwater doesn’t. What results looks sort of like a heat map, showing regions where fresh and saltwater are concentrated.

Person compared the electromagnetic and isotope data with his models to see which historical scenarios (ice or no ice) were statistically likely to form an aquifer that matched all the data. His results, which are in the review stage with the Geological Society of America Bulletin, suggest it’s very likely that ice sheets played a role in forming the aquifer.

“There’s a lot of uncertainty,” Person said, but “it’s the best thing we have going.”

East Coast has a giant offshore freshwater aquifer—how did it get there? Read More »

Climate damages by 2050 will be 6 times the cost of limiting warming to 2°

adaptation, climate change, Earth science, Economics, green, mitigation, Science, Sustainability / Mike M. / April 18, 2024

A worker walks between long rows of solar panels.

Almost from the start, arguments about mitigating climate change have included an element of cost-benefit analysis: Would it cost more to move the world off fossil fuels than it would to simply try to adapt to a changing world? A strong consensus has built that the answer to the question is a clear no, capped off by a Nobel in Economics given to one of the people whose work was key to building that consensus.

While most academics may have considered the argument put to rest, it has enjoyed an extended life in the political sphere. Large unknowns remain about both the costs and benefits, which depend in part on the remaining uncertainties in climate science and in part on the assumptions baked into economic models.

In Wednesday’s edition of Nature, a small team of researchers analyzed how local economies have responded to the last 40 years of warming and projected those effects forward to 2050. They find that we’re already committed to warming that will see the growth of the global economy undercut by 20 percent. That places the cost of even a limited period of climate change at roughly six times the estimated price of putting the world on a path to limit the warming to 2° C.

Linking economics and climate

Many economic studies of climate change involve assumptions about the value of spending today to avoid the costs of a warmer climate in the future, as well as the details of those costs. But the people behind the new work, Maximilian Kotz, Anders Levermann, and Leonie Wenz decided to take an empirical approach. They obtained data about the economic performance of over 1,600 individual regions around the globe, going back 40 years. They then attempted to look for connections between that performance and climate events.

Previous research already identified a number of climate measures—average temperatures, daily temperature variability, total annual precipitation, the annual number of wet days, and extreme daily rainfall—that have all been linked to economic impacts. Some of these effects, like extreme rainfall, are likely to have immediate effects. Others on this list, like temperature variability, are likely to have a gradual impact that is only felt over time.

The researchers tested each factor for lagging effects, meaning an economic impact sometime after their onset. These suggested that temperature factors could have a lagging impact up to eight years after they changed, while precipitation changes were typically felt within four years of climate-driven changes. While this relationship might be in error for some of the economic changes in some regions, the inclusion of so many regions and a long time period should help limit the impact of those spurious correlations.

With the climate/economic relationship worked out, the researchers obtained climate projections from the Coupled Model Intercomparison Project (CMIP) project. With that in hand, they could look at future climates and estimate their economic costs.

Obviously, there are limits to how far into the future this process will work. The uncertainties of the climate models grow with time; the future economy starts looking a lot less like the present, and things like temperature extremes start to reach levels where past economic behavior no longer applies.

To deal with that, Kotz, Levermann, and Wenz performed a random sampling to determine the uncertainty in the system they developed. They look for the point where the uncertainties from the two most extreme emissions scenarios overlap. That occurs in 2049; after that, we can’t expect the past economic impacts of climate to apply.

Kotz, Levermann, and Wenz suggest that this is an indication of warming we’re already committed to, in part because the effect of past emissions hasn’t been felt in its entirety and partly because the global economy is a boat that turns slowly, so it will take time to implement significant changes in emissions. “Such a focus on the near term limits the large uncertainties about diverging future emission trajectories, the resulting long-term climate response and the validity of applying historically observed climate–economic relations over long timescales during which socio-technical conditions may change considerably,” they argue.

Climate damages by 2050 will be 6 times the cost of limiting warming to 2° Read More »

What would the late heavy bombardment have done to the Earth’s surface?

asteroids, craters, Earth science, geology, Impacts, planetary science, Science, tsunamis / DJ Henderson / January 26, 2024

Under fire —

Early in Earth’s history, bombardment by enormous asteroids was common.

Alka Tripathy-Lang – Jan 26, 2024 6: 38 pm UTC

Image of a projection of the globe, with multi-colored splotches covering its surface. — Enlarge / Each panel shows the modeled effects of early Earth’s bombardment. Circles show the regions affected by each impact, with diameters corresponding to the final size of craters for impactors smaller than 100 kilometers in diameter. For larger impactors, the circle size corresponds to size of the region buried by impact-generated melt. Color coding indicates the timing of the impacts. The smallest impactors considered in this model have a diameter of 15 kilometers.

Simone Marchi, Southwest Research Institute

When it comes to space rocks slamming into Earth, two stand out. There’s the one that killed the dinosaurs 65 million years ago (goodbye T-rex, hello mammals!) and the one that formed Earth’s Moon. The asteroid that hurtled into the Yucatan peninsula and decimated the dinosaurs was a mere 10 kilometers in diameter. The impactor that formed the Moon, on the other hand, may have been about the size of Mars. But between the gigantic lunar-forming impact and the comparatively diminutive harbinger of dinosaurian death, Earth was certainly battered by other bodies.

At the 2023 Fall Meeting of the American Geophysical Union, scientists discussed what they’ve found when it comes to just how our planet has been shaped by asteroids that impacted the early Earth, causing everything from voluminous melts that covered swaths of the surface to ancient tsunamis that tore across the globe.

Modeling melt

When the Moon-forming impactor smashed into Earth, much of the world became a sea of melted rock called a magma ocean (if it wasn’t already melted). After this point, Earth had no more major additions of mass, said Simone Marchi, a planetary scientist at the Southwest Research Institute who creates computer models of the early Solar System and its planetary bodies, including Earth. “But you still have this debris flying about,” he said. This later phase of accretion may have lacked another lunar-scale impact, but likely featured large incoming asteroids. Predictions of the size and frequency distributions of this space flotsam indicate “that there has to be a substantial number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.

Unfortunately, there’s little obvious evidence in the rock record of these impacts before about 3.5 billion years ago. So scientists like Marchi can look to the Moon to estimate the number of objects that must have collided with Earth.

Armed with the size and number of impactors, Marchi and colleagues built a model that describes, as a function of time, the volume of melt this battering must have produced at the Earth’s surface. Magma oceans were in the past, but impactors greater than 100 kilometers in diameter still melted a lot of rock and must have drastically altered the early Earth.

Unlike smaller impacts, the volume of melt generated by objects of this size isn’t localized within a crater, according to models. Any crater exists only momentarily, as the rock is too fluid to maintain any sort of structure. Marchi compares this to tossing a stone into water. “There is a moment in time in which you have a cavity in the water, but then everything collapses and fills up because it’s a fluid.”

The melt volume is much larger than the amount of excavated rock, so Marchi can calculate just how much melt might have spilled out and coated parts of the Earth’s surface with each impact. The result is an astonishing map of melt volume. During the first billion years or so of Earth’s history, nearly the entire surface would have featured a veneer of impact melt at some point. Much of that history is gone because our active planet’s atmospheric, surface, and tectonic processes constantly modify much of the rock record.

Balls of glass

Even between 3.5 and 2.5 billion years ago, the rock record is sparse. But two places, Australia and South Africa, preserve evidence of impacts in the form of spherules. These tiny glass balls form immediately after an impact that sends vaporized rock skyward. As the plume returns to Earth, small droplets begin to condense and rain down.

<span class= — Enlarge / Spherule bed from impact S3 in drill core. Here, S3’s spherule beds were deposited in deep enough water to not be diluted by other detritus.

Nadja Drabon, Harvard

“It’s remarkable that we can find these impact-generated spherule layers all the way back to 3.5 billion years ago,” said Marchi.

What would the late heavy bombardment have done to the Earth’s surface? Read More »