Max Planck Institute for Meteorology

Hamburg, Germany

Max Planck Institute for Meteorology

Hamburg, Germany
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News Article | April 20, 2017

New insights into the impact forests have on surface temperature will provide a valuable tool in efforts to mitigate climate change, according to a new research paper co-authored by Clemson University scientist Thomas O'Halloran. For the first time, scientists have created a global map measuring the cooling effect forests generate by regulating the exchange of water and energy between the Earth's surface and the atmosphere. In many locations, this cooling effect works in concert with forests' absorption of carbon dioxide. By coupling information from satellites with local data from sensors mounted to research towers extending high above tree canopies, O'Halloran and his collaborators throughout the world have given a much more complete, diagnostic view of the roles forests play in regulating climate. Their findings have important implications for how and where different types of land cover can be used to mitigate climate change with forest protection programs and data-driven land-use policies. Results of their study were recently published in the journal Nature Climate Change. "It's our hope that such global maps can be used to optimize biophysics in addition to carbon when planning land-use climate change mitigation projects," said O'Halloran, assistant professor of Forestry and Environmental Conservation at Clemson's Baruch Institute of Coastal Ecology and Forest Science in Georgetown. Previously, scientists measured vegetation's impact on local land temperatures using satellite imagery, which is limited to only clear-sky days and few measurements per day, or they used local stations, which are limited in their reach. Integrating data from towers extending more than 100 feet in the air with satellite measurements allows for a more advanced view of the variables impacting surface temperature. The research team found that forests' cooling effect was greater than thought and most pronounced in mid- and low-latitude regions. This new statistical model of analyzing forests' impact on local temperature will allow communities around the world to pinpoint ideal locations for forest protection or reforestation efforts. "We wanted every country in the world to have some estimation of the cooling effects of forests and vegetation," O'Halloran said. "It's about optimizing the benefit of land management for climate change mitigation." A tower similar to those used for this study is under construction at Baruch in collaboration with the University of South Carolina to help provide greater analysis of local climate, he said. "The towers will really help us understand how ecosystems respond to change," O'Halloran said. "In South Carolina, we've had a lot of extreme weather events, droughts, flood and hurricanes. This will help us understand the resilience of local ecosystems to those types of events." O'Halloran co-authored the article in Nature Climate Change with lead author Ryan Bright of The Norwegian Institute of Bioeconomy Research in Norway and several additional collaborators throughout Europe and the United States. Unlike local climate changes owed to global emissions of CO2 and other greenhouse gases, local climate changes linked to land-related activities are unique in that they are only influenced by the local land-use policies that are in place, Bright said. "The results of our study now make it easier for individual nations or regions to begin measuring and enforcing climate policies resulting in tangible mitigation or adaptation benefits at the local scale," says Bright. "This is especially critical moving forward in a world facing increasing competition for land resources." Other research collaborators were Edouard Davin of the Institute for Atmospheric and Climate Science in Switzerland; Julia Pongratz of the Max Planck Institute for Meteorology in Germany; Kaiguang Zhao of the School of Environment and Natural Resources at The Ohio State University; and Alessandro Cescatti of the European Commission's Joint Research Centre in Italy.

News Article | May 22, 2017

Scientists are expressing increasing skepticism that we’re going to be able to get out of the climate change mess by relying on a variety of large-scale land-use and technical solutions that have been not only proposed but often relied upon in scientific calculations. Two papers published last week debunk the idea of planting large volumes of trees to pull carbon dioxide out of the air — saying there just isn’t enough land available to pull it off — and also various other strategies for “carbon dioxide removal,” some of which also include massive tree plantings combined with burning their biomass and storing it below the ground. “Biomass plantations are always seen as a green kind of climate engineering because, you know, everybody likes trees,” said Lena Boysen, a climate researcher at the Max Planck Institute for Meteorology in Germany, who led one of the new studies while a researcher at the Potsdam Institute for Climate Impact Research. “But we just want to show that that’s not the complete story. They cannot do that much.” Forests have long been recognized as one of the world’s most important natural carbon sinks, capable of storing large amounts of carbon that would otherwise end up in the atmosphere. Simply preserving the world’s forest resources — and replanting areas that have already been deforested — is viewed as an important step in protecting the climate. But for years, scientists have discussed the idea of going further by using large plantations full of fast-growing, carbon-storing trees to pull extra carbon emissions out of the atmosphere, a strategy sometimes called “afforestation.” But the amount of land and other resources this strategy would require to actually help us meet our global climate goals — namely, keeping global temperatures within at least two degrees of their pre-industrial levels — is completely impractical, according to Boysen’s new study in the journal Earth’s Future, and would require the destruction of huge amounts of natural ecosystems or productive agricultural land. Considering different scenarios for planetary emissions, Boysen and her colleagues find the land space that would be required for the amount of trees necessary to keep temperatures within a 2-degree threshold under our current climate trajectory could have “dire consequences for food production or the biosphere.” And even under more optimistic scenarios, where future carbon emissions are lower and fewer trees would be necessary, they conclude that “high inputs of managed water and fertilizers would be needed in order to avoid fierce competition for land — with potentially negative side-effects for climate and society.” It isn’t just planting forests on their own: Many scientists have suggested that a more advanced strategy may be necessary to keep to a two-degree pathway in the future — particularly now, as analyses increasingly suggest that even the emissions reduction pledges established by countries participating in the Paris climate agreement are still not enough to keep us on track. The solution that’s been proposed in numerous reports and climate models, including those released over the years by the Intergovernmental Panel on Climate Change (IPCC), is a technology known as bioenergy and carbon capture and storage, or BECCS. This strategy involves establishing large plantations of fast-growing trees, capable of storing large quantities of carbon, which can then be harvested and used for fuel. Biomass burning facilities would need to be outfitted with a special carbon-capturing technology, which would capture the carbon dioxide produced and store it safely away, potentially in geological formations deep underground. It’s an ambitious proposal, and one that many scientists have pointed out is nowhere near the point of becoming feasible, even from a technological perspective. Carbon capture and storage technology is just getting on its feet from a commercial standpoint, and there are only a few facilities around the world — several of them in the United States — that use it. Without this technology, the planting and harvesting of biomass becomes far less climate friendly, essentially releasing all the stored carbon right back into the atmosphere. These biomass energy systems could be valuable, “but only if the biomass is entirely sustainable,” said Daniel Kammen, an energy professor at the University of California at Berkeley and director of the Renewable and Appropriate Energy Laboratory, in an emailed comment to The Washington Post. Kammen was not involved with the new study but has conducted much of his own research on biomass energy and carbon capture and storage systems. The new study, he says, agrees well with what previous work has suggested — that when the system is not completely sustainable, “the value of a biomass crop for meeting climate targets is non-existent,” he said. Large plantations of fast-growing trees, as explored in the recent paper, are clearly not sustainable, he pointed out. And other experts agree as well. In a separate paper, published Thursday in the journal Science, Stanford researchers Christopher Field and Katharine Mach argue that carbon dioxide removal strategies, including BECCS, are still too risky to rely on for limiting climate change. In regard to BECCS, they also point out that the massive amount of resources required to sustain the system makes it unfeasible. “Converting land on this staggering scale would pit climate change responses against food security and biodiversity protection,” they write. “Massively expanding managed land for CDR [carbon dioxide removal] could crash through the planetary boundary for sustainable land use.” Worryingly, they add, carbon dioxide removal is increasingly assumed by climate models and planning tools as a future mitigation tactic. While they encourage continued research and development of the technology, the authors also urge the necessity of “avoiding cavalier assumptions of future technological breakthroughs.” That’s not to say that there isn’t any place for this type of technology in the future, Boysen added. “There are areas indeed where we can plant biomass plantations where they would be beneficial for the region,” she pointed out. But on a global scale, she said, research suggests that it’s just not an effective mitigation tool. “There are other options,” she said, such as more sustainable agriculture techniques, that can help make a dent in global carbon emissions. “But the most important option is, of course, not to release the CO2 into the atmosphere.”

News Article | May 23, 2017

"Scientists are expressing increasing skepticism that we’re going to be able to get out of the climate change mess by relying on a variety of large-scale land-use and technical solutions that have been not only proposed but often relied upon in scientific calculations. Two papers published last week debunk the idea of planting large volumes of trees to pull carbon dioxide out of the air — saying there just isn’t enough land available to pull it off — and also various other strategies for “carbon dioxide removal,” some of which also include massive tree plantings combined with burning their biomass and storing it below the ground. “Biomass plantations are always seen as a green kind of climate engineering because, you know, everybody likes trees,” said Lena Boysen, a climate researcher at the Max Planck Institute for Meteorology in Germany, who led one of the new studies while a researcher at the Potsdam Institute for Climate Impact Research. “But we just want to show that that’s not the complete story. They cannot do that much.”" Chelsea Harvey reports for the Washington Post May 22, 2017.

Mellado J.P.,Max Planck Institute for Meteorology
Annual Review of Fluid Mechanics | Year: 2017

Cloud entrainment, the mixing between cloudy and clear air at the boundary of clouds, constitutes one paradigm for the relevance of small scales in the Earth system: By regulating cloud lifetimes, meter- and submeter-scale processes at cloud boundaries can influence planetary-scale properties. Understanding cloud entrainment is difficult given the complexity and diversity of the associated phenomena, which include turbulence entrainment within a stratified medium, convective instabilities driven by radiative and evaporative cooling, shear instabilities, and cloud microphysics. Obtaining accurate data at the required small scales is also challenging, for both simulations and measurements. During the past few decades, however, high-resolution simulations and measurements have greatly advanced our understanding of the main mechanisms controlling cloud entrainment. This article reviews some of these advances, focusing on stratocumulus clouds, and indicates remaining challenges. © Copyright 2017 by Annual Reviews. All rights reserved.

Stevens B.,Max Planck Institute for Meteorology
Journal of Climate | Year: 2015

Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre-1950) part of the instrumental record. This model, which contains terms representing both aerosol-radiation and aerosol-cloud interactions, well represents the known time history of aerosol radiative forcing as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrate that a forcing more negative than -1.0Wm-2 is implausible, as it implies that none of the approximately 0.3-K temperature rise between 1850 and 1950 can be attributed to Northern Hemisphere forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding to provide further support for the less negative (-1.0Wm-2) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed. © 2015 American Meteorological Society.

Marotzke J.,Max Planck Institute for Meteorology | Forster P.M.,University of Leeds
Nature | Year: 2015

Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors. Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST - with the regression residual interpreted as internal variability - and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations. The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded. © 2015 Macmillan Publishers Limited. All rights reserved.

Pithan F.,Max Planck Institute for Meteorology | Mauritsen T.,Max Planck Institute for Meteorology
Nature Geoscience | Year: 2014

Climate change is amplified in the Arctic region. Arctic amplification has been found in past warm and glacial periods, as well as in historical observations and climate model experiments. Feedback effects associated with temperature, water vapour and clouds have been suggested to contribute to amplified warming in the Arctic, but the surface albedo feedback - the increase in surface absorption of solar radiation when snow and ice retreat - is often cited as the main contributor. However, Arctic amplification is also found in models without changes in snow and ice cover. Here we analyse climate model simulations from the Coupled Model Intercomparison Project Phase 5 archive to quantify the contributions of the various feedbacks. We find that in the simulations, the largest contribution to Arctic amplification comes from a temperature feedbacks: as the surface warms, more energy is radiated back to space in low latitudes, compared with the Arctic. This effect can be attributed to both the different vertical structure of the warming in high and low latitudes, and a smaller increase in emitted blackbody radiation per unit warming at colder temperatures. We find that the surface albedo feedback is the second main contributor to Arctic amplification and that other contributions are substantially smaller or even opposeArctic amplification. © 2014 Macmillan Publishers Limited.

Notz D.,Max Planck Institute for Meteorology
Cryosphere | Year: 2014

We examine how the evaluation of modelled sea-ice coverage against reality is affected by uncertainties in the retrieval of sea-ice coverage from satellite, by the usage of sea-ice extent to overcome these uncertainties, and by internal variability. We find that for Arctic summer sea ice, model biases in sea-ice extent can be qualitatively different from biases in sea-ice area. This is because about half of the CMIP5 models and satellite retrievals based on the Bootstrap and the ASI algorithm show a compact ice cover in summer with large areas of high-concentration sea ice, while the other half of the CMIP5 models and satellite retrievals based on the NASA Team algorithm show a loose ice cover. For the Arctic winter sea-ice cover, differences in grid geometry can cause synthetic biases in sea-ice extent that are larger than the observational uncertainty. Comparing the uncertainty arising directly from the satellite retrievals with those that arise from internal variability, we find that the latter by far dominates the uncertainty estimate for trends in sea-ice extent and area: most of the differences between modelled and observed trends can simply be explained by internal variability. For absolute sea-ice area and sea-ice extent, however, internal variability cannot explain the difference between model and observations for about half the CMIP5 models that we analyse here. All models that we examined have regional biases, as expressed by the root-mean-square error in concentration, that are larger than the differences between individual satellite algorithms.

Seifert A.,Hans Ertel Center for Weather Research | Heus T.,Max Planck Institute for Meteorology
Atmospheric Chemistry and Physics | Year: 2013

Trade wind cumulus clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual clouds as well as the mesoscale organization on scales of O(10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulation experiments, the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization. © 2013 Author(s).

Notz D.,Max Planck Institute for Meteorology | Marotzke J.,Max Planck Institute for Meteorology
Geophysical Research Letters | Year: 2012

The very low summer extent of Arctic sea ice that has been observed in recent years is often casually interpreted as an early-warning sign of anthropogenic global warming. For examining the validity of this claim, previously IPCC model simulations have been used. Here, we focus on the available observational record to examine if this record allows us to identify either internal variability, self-acceleration, or a specific external forcing as the main driver for the observed sea-ice retreat. We find that the available observations are sufficient to virtually exclude internal variability and self-acceleration as an explanation for the observed long-term trend, clustering, and magnitude of recent sea-ice minima. Instead, the recent retreat is well described by the superposition of an externally forced linear trend and internal variability. For the externally forced trend, we find a physically plausible strong correlation only with increasing atmospheric CO 2 concentration. Our results hence show that the observed evolution of Arctic sea-ice extent is consistent with the claim that virtually certainly the impact of an anthropogenic climate change is observable in Arctic sea ice already today. Copyright 2012 by the American Geophysical Union.

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