Please quote Nature Geoscience as the source of these items.
The atmosphere’s tug on Venus’s rotation
Venus's spin changes speed as its dense, fast-flowing atmosphere interacts with mountains on the planet's surface, reports a study published online this week in Nature Geoscience.
Venus rotates slowly, with one revolution taking about 243 Earth days. However, measurements by visiting spacecraft have not agreed on the precise length of a Venusian day. One reason might be the giant bow-shaped atmospheric structure recently spotted appearing and disappearing by the Akatsuki spacecraft. Despite the Venusian atmosphere moving much faster than the planet itself (completing one rotation in four Earth days), the structure remained stationary above a mountainous region. It has been suggested that this structure was an atmospheric wave, caused by the lower atmosphere rising over mountain topography If correct, the atmosphere and solid planet might be more closely linked than originally thought.
To test this hypothesis, Thomas Navarro and colleagues simulate the circulation of the Venusian atmosphere. They find that the bow-shaped structure can indeed be explained by atmospheric waves forming over the mountains, with the waves only forming in the afternoon and vanishing by dusk. The authors also find that the formation of these waves causes atmospheric pressure fluctuations that actually change the rotation rate of the solid planet, depending on the time of day.
The authors find that this effect is small — a change that would alter the length of a Venusian day by only a couple of minutes — but this interplay between the solid planet and its atmosphere may explain at least a part of the discrepancies between past measurements of Venus’s rotation rate.
Size matters for Amazonian trees during dry spells
Taller trees in tropical forests are more resilient to drought, reports a paper published online this week in Nature Geoscience.
Severe droughts in the Amazon jungle — a potential consequence of global climate change — could lead to widespread forest losses. It is not known, however, what role different tree attributes, such as height, play in the response of forests to drought.
Pierre Gentine and colleagues analysed how sensitive the photosynthesis of tall and short trees in Amazon forests is to drought. They find that tall trees above 30 metres are three times less sensitive to drought than short trees under 20 metres. Taller trees have more extensive root systems that allow them to reach deep soil moisture during dry seasons. They are, however, more vulnerable to atmospheric aridity. Leaves on tall trees constantly have a lower water content, which makes their photosynthesis more adaptable to soil drought but also more sensitive to fluctuations in atmospheric water.
The authors conclude that forest height may play as important a role as precipitation levels in regulating photosynthesis to cope with climate variations.
Multiple migrations of the Great Barrier Reef
The Great Barrier Reef has migrated back and forth across the ocean floor in response to large sea-level fluctuations over the past 30,000 years, reports a paper published online this week in Nature Geoscience.
Over the past 30,000 years, sea level has varied dramatically as continental ice sheets expanded and waned. At the Last Glacial Maximum, about 21,000 years ago, sea level was about 120 metres lower than it is today.
Jody Webster and colleagues examined drill cores collected near the modern Great Barrier Reef to assess how the reef system responded to past sea-level changes. They identified five reef death events: two caused by the exposure of the reef as sea level fell, and three caused by drowning as sea level rose faster than the corals could grow. The final death event was hastened by high sediment fluxes 10,000 years ago, and led to the establishment of the reef in its modern location. The authors also found that, following each event, the reef was able to re-establish at a more suitable depth within hundreds to thousands of years. This suggests that the reef’s long-term resilience was supported by connections with nearby reefs that could repopulate the reef as it migrated.
Carbon budgets must be reconsidered
The usefulness of a fixed carbon budget to keep global warming below a prescribed temperature level is questioned in two Comment pieces published online this week in Nature Geoscience. The authors argue that instead of focusing on a single number, it would be more fruitful to focus on emissions ranges and political realities in climate policy discussions.
The concept that the level of warming ultimately only depends on total carbon emissions, and not on the evolution of emissions over time, was introduced in 2009 in the run-up to the Copenhagen Climate Summit. As a result, a carbon budget can be calculated for given temperature levels, such as the 1.5 and 2 °C targets written into the Paris Agreement.
In the first piece, Glen Peters argues that carbon budgets are of limited use for policymakers for two main reasons. First, their scale is inherently global, whereas policy decisions are made on a country-by-country scale; and second, they are highly uncertain, because of incomplete knowledge of the carbon cycle and because carbon budget estimates also depend on user and societal choices. He suggests that there is a need to openly discuss these uncertainties and focus on the goal to reach zero emissions in the second half of the century.
In the second Comment, Oliver Geden notes that since its introduction in 2009, the concept of carbon budgets has only influenced climate policy talk, but not climate policy actions. He suggests that a different framing of the communication from the climate scientists would be more aligned with initiating action under political realities. Instead of continuing with a narrative that it is ‘five minutes to midnight’, scientists could point out that achieving climate targets of 1.5 or 2 °C is implausible, unless stringent action is taken.
Earth’s early pulse formed first stable crust
Earth’s oldest stable crust may have formed during multiple pulses of overturning, reports a study published online this week in Nature Geoscience.
Continental crust today is formed through plate tectonics, mostly via the volcanic activity that occurs where one plate subducts under another. In the Archaean eon (4 to 2.6 billion years ago), however, Earth was likely probably too hot to sustain modern-style plate tectonics. Instead, continental crust may have formed through gravitational instabilities. In this scenario, a mixture of dense and lighter minerals initially separated from the mantle, crystallizing to form a proto-crust. The denser components were gravitationally unstable, causing large chunks of crust to overturn and be recycled back into the mantle. This process repeated until lower-density components were concentrated at the surface, forming a stable crust. Some have proposed that crust formed in this way is preserved in a craton — an old and geologically stable region of continental crust — in Western Australia’s Pilbara region.
Daniel Wiemer and colleagues dated the rocks in the East Pilbara Craton and used a thermodynamic model to reconstruct the purported gravitational overturns. They propose that the rocks formed around 3.6 to 3.4 billion years ago during an overturn event that lasted about 100 million years. The researchers also use existing data to argue that similar events are recorded in cratons in India and Africa. They suggest that to create stable continental crust in each craton, three cycles of gravitational overturn, each lasting about 100 million years, would have been required. These cycles may therefore represent the ‘pulse’ of the early Earth.
Floods flush away microplastic pollution from riverbeds
Floods can flush out large quantities of microplastic pollution from riverbeds, reports a study published online this week in Nature Geoscience.
Microplastics are plastic particles that are less than 5 mm in length. Some are deliberately manufactured for the cosmetics industry, while others form when larger plastics break apart. Microplastics contaminate the world’s oceans, threatening both the environment and ecosystem. Although approximately 90% of microplastic contamination in the oceans is thought to originate from land, not much is known about the role rivers play in this process.
Rachel Hurley and colleagues examined the microplastics in river sediments from 40 sites across northwest England. They found microplastics in all riverbeds, even in the smallest rural streams, and identified various urban pollution hotspots with concentrations of up to 517,000 plastic particles per square metre. After severe flooding in the winter of 2015/16, the authors sampled the same locations again. They found that the microplastic pollution had decreased at 70% of the sites. The flooding transported away around 70% of the total microplastic load from the riverbeds —amounting to about 0.85 tonnes of plastic —and flushed all microbead pollution from seven of the sites.
Soil biocrusts vulnerable to changing climate and land-use
The area covered by biocrusts—the algae, lichens, and mosses that currently cover 18 million km2 of soil across 12% of Earth’s total land surface—could decrease by about 25–40% by 2070 due to climate and land-use change, reports a study published online this week in Nature Geoscience.
Biological soil crusts, or biocrusts, are soil surface communities that help control the movement of water into soils and, by extension, affect nutrient cycling and local biodiversity. They also stabilise the soil surface, limiting dust generation. Biocrusts are mostly found in dry regions, where larger plants are not present to out-compete them.
Emilio Rodriguez-Caballero and colleagues looked at more than 500 previously published biocrust studies. They identified 18 different factors that make certain areas more suitable than others for forming biocrusts. Based on this classification, combined with climate and land-use projections, the authors infer a future decrease in the availability of land suitable for biocrust growth. Verifying their results with different models, they find that the area covered by biocrusts could decrease by up to 40% by 2070.
Reduction in Northern Hemisphere mid-latitude wind energy resource with global warming
The amount of wind available for converting into electricity — key to reducing carbon emissions — is projected to decrease in the Northern Hemisphere mid-latitudes as the climate warms. The study, published online this week in Nature Geoscience, finds that this will particularly affect the central United States, the United Kingdom and Ireland, the northern Middle East, and central, northern and far eastern Asia. However, it finds robust increases in the wind available to convert into energy for tropical and Southern Hemisphere regions under high-emissions scenarios.
The capacity to generate power from wind farms is growing rapidly around the globe, as a strategy to reduce the dependence on energy from fossil fuels. However, regional studies have shown that the wind energy resource — the amount of wind available for converting into energy using turbines — can be subject to change in a changing climate.
Kristopher Karnauskas and colleagues combine global climate model simulations with an industry wind power turbine curve to derive the impact of projected changes in climate on future wind power capacity. They find that wind power averaged over the central US, for example, is projected to decrease by 8-10% by 2050 and 14-18% by 2100, depending on the emissions scenario. This decline in wind power can be explained by the rapid warming in the Arctic, which reduces the temperature difference between the Arctic and the tropics that ultimately drives the intensity of storms.
Lower roadside NO2 emissions than assumed
The proportion of nitrogen dioxide (NO2) in European roadside nitrogen oxides (NOx) emissions could be two times smaller than assumed in policy projections, concludes a study published online in Nature Geoscience this week. The study therefore suggests that European roadside air quality standards — which regulate NO2 concentrations — may be achieved sooner than expected.
In Europe, emissions standards for new vehicles regulate nitrogen oxides (NOx), which encompasses NO2 and nitric oxide (NO). These emissions have declined in Europe since about 1990. By contrast, air quality standards set limits for the concentration of NO2 only, and roadside concentrations of NO2 have declined much less than expected, which has been largely attributed to increasing use of diesel vehicles in Europe and more direct tail-pipe emissions of NO2.
Stuart Grange and colleagues analysed 130 million hourly measurements of NOx, NO2 and ozone (O3) from roadside monitoring stations across 61 European cities. They find that that the proportion of NO2 in NOx increased from 1995 to around 2010, but stabilized or decreased thereafter in most regions, and remained at a level that is lower than assumed in air quality standards. The authors suggest that policy projections of air quality that use too high a value for the proportion of NO2 in NOx will predict higher concentrations of roadside NO2 than may actually occur for the same total amount of NOx emitted.
In an accompanying News & Views article, Drew Gentner writes: “Going forward, the findings are important for informing air-quality policies in diesel-vehicle-heavy developing regions, which typically inherit used vehicles and have emissions standards that lag slightly behind the US and Europe."
Temperature overshoot scenarios need limits
Aiming for the more ambitious Paris Agreement climate goal of limiting human-induced warming to 1.5 °C rather than 2 °C could unintentionally weaken climate policy, argues a Comment published online this week in Nature Geoscience. The authors suggest this goal could make the concept that a temperature target can be overshot for some time acceptable, and that such a relaxation could become a source for political flexibility, unless strict limitations on the temperature pathway to achieve the goal are implemented.
Previous research has suggested that meeting the 1.5 °C temperature goal that the parties to the Paris Agreement aspire to will most likely only be possible by allowing temporary warming in excess of that temperature, followed by aggressive mitigation to compensate.
However, Oliver Geden and Andreas Löschel write that broad acceptance for such a temperature overshoot scenario could undermine the perception of temperature targets as firm limits to warming. They suggest that climate scientists must define clear constraints to the acceptable magnitude, duration and end time for any period of overshoot, if they wish to ensure that mitigation targets are precise and evaluable, and to preserve political accountability.
Storms track further poleward under climate change
In a warming climate, mid-latitude storms will travel further towards the poles before they reach their maximum intensity, with potentially severe implications for mid-latitude weather, reports a paper published online this week in Nature Geoscience. The study suggests that impacts on local weather and climate are likely to be strongest in regions that are close to the northeastern ocean boundaries, such as the United States west coast and Britain.
Low-pressure storms outside the tropics are often associated with intense precipitation and wind. Based on climate model simulations, their tracks are expected to shift poleward in a changing climate, but the dominant underlying mechanisms are debated.
Talia Tamarin-Brodsky and Yohai Kaspi analysed simulations from 20 state-of-the-art climate models with an algorithm that allowed them to identify the tracks of individual storms. They found that a significant part of the poleward shift of storm activity can be attributed to a longer propagation distance, which they suggest is caused by stronger winds at the upper level of the atmosphere and increased concentrations of atmospheric water vapour.
Optimization of crop distributions to feed millions and save water
An additional 825 million people around the world could be fed, while also saving water resources, if the agricultural landscape is configured to optimize the distribution of crops, concludes a study published online this week in Nature Geoscience. Although the application of such an optimization process may be limited to some extent by cultural barriers and dietary preferences, it has the advantage of not requiring large investment in technology or implying a loss of biodiversity, making it a viable strategy for sustainable intensification of agriculture.
With growth in global population, demand for richer diets and use of biofuel expected for the coming decades, crop production will need to increase. Yet, given finite resources, sustainable intensification will require enhanced production on existing farmland, whilst seeking to preserve resources such as water, fertilizer and energy.
Kyle Davis and colleagues used a crop water model together with maps of 14 major food crop yields to investigate the potential of increasing food production through a global spatial redistribution of crops on cultivated land. The authors identify crop configurations that produce 10% more calories and 19% more protein, and suggest that crop redistribution can help nations reduce their dependence on food imports. These re-configurations of crops would also reduce the consumptive use of rainwater by 14% and irrigation water by 12%.
Increased food production and reduced water use through optimized crop distribution
Ancient Mars thawed by methane bursts
Ancient Mars may have been warmed by explosive bursts of methane trapped in its subsurface, according to a study published online in Nature Geoscience this week. This finding explains the evidence of episodic wet climates on an otherwise freezing planet.
Sediments on Mars — such as those currently being explored by the Curiosity rover at Gale crater — suggest that lakes of liquid water existed on Mars later than three billion years ago, despite evidence that Mars was largely cold and dry at this time. It has been difficult to explain how sufficient climate warming occurred to allow these lakes to form, and also how warm climates lasted long enough to allow the lakes to persist for several thousand years, as geological evidence suggests.
Edwin Kite and colleagues use numerical simulations to propose that warming due to release of the potent greenhouse gas methane into the atmosphere can explain both the existence and persistence of the lakes. In this scenario, shifts in the tilt of the planet’s axis can cause the planet’s ice cover to shrink. Like on Earth, methane can be stored in Martian soils and trapped beneath ice. If the ice cover is removed, the stored methane is destabilized, causing the methane to explosively erupt into the atmosphere. The authors calculate that sufficient methane could have been released by this process to produce a lake-forming climate on ancient Mars. Although the methane would gradually break down in the Martian atmosphere, they estimate that each warming episode would persist for up to a million years.
In an accompanying News & Views article, Alberto Fairén writes that: “The methane burst scenario proposed by Kite et al. contributes to an emerging view that the existence of liquid water on early Mars arose from a combination of diverse astronomical, geochemical and geological factors.”
Methane bursts as a trigger for intermittent lake-forming climates on post-Noachian Mars
Planetary science: Icy Mars lakes warmed by methane
Alberto G. Fairén
Published online: 2 October 2017 | doi 10.1038/ngeo3037