Blog Archive

Monday, January 15, 2018

Detecting the permafrost carbon feedback: talik formation and increased cold-season respiration as precursors to sink-to-source transitions

The Cryosphere, 12(1) (2018) 123144

Detecting the permafrost carbon feedback: Talik formation and increased cold-season respiration as precursors to sink-to-source transitions

Nicholas C. Parazoo1, Charles D. Koven2, David M. Lawrence3, Vladimir Romanovsky4, and Charles E. Miller1
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA
2Lawrence Berkeley National Laboratory, Berkeley, CA, USA
3National Center for Atmospheric Research, Boulder, CO, USA
4Geophysical Institute UAF, Fairbanks, AK, 99775, USA

Received: 31 Aug 2017; discussion started: 18 Sep 2017
Revised: 20 Nov 2017; accepted: 29 Nov 2017; published: 12 Jan 2018


Thaw and release of permafrost carbon (C) due to climate change is likely to offset increased vegetation C uptake in northern high-latitude (NHL) terrestrial ecosystems. Models project that this permafrost C feedback may act as a slow leak, in which case detection and attribution of the feedback may be difficult. The formation of talik, a subsurface layer of perennially thawed soil, can accelerate permafrost degradation and soil respiration, ultimately shifting the C balance of permafrost-affected ecosystems from long-term C sinks to long-term C sources. It is imperative to understand and characterize mechanistic links between talik, permafrost thaw, and respiration of deep soil C to detect and quantify the permafrost C feedback. Here, we use the Community Land Model (CLM) version 4.5, a permafrost and biogeochemistry model, in comparison to long-term deep borehole data along North American and Siberian transects, to investigate thaw-driven C sources in NHL ( >  55° N) from 2000 to 2300. Widespread talik at depth is projected across most of the NHL permafrost region (14 million km2) by 2300, 6.2 million km2 of which is projected to become a long-term C source, emitting 10 Pg C by 2100, 50 Pg C by 2200, and 120 Pg C by 2300, with few signs of slowing. Roughly half of the projected C source region is in predominantly warm sub-Arctic permafrost following talik onset. This region emits only 20 Pg C by 2300, but the CLM4.5 estimate may be biased low by not accounting for deep C in yedoma. Accelerated decomposition of deep soil C following talik onset shifts the ecosystem C balance away from surface dominant processes (photosynthesis and litter respiration), but sink-to-source transition dates are delayed by 20–200 years by high ecosystem productivity, such that talik peaks early ( ∼  2050s, although borehole data suggest sooner) and C source transition peaks late ( ∼  2150–2200). The remaining C source region in cold northern Arctic permafrost, which shifts to a net source early (late 21st century), emits 5 times more C (95 Pg C) by 2300, and prior to talik formation due to the high decomposition rates of shallow, young C in organic-rich soils coupled with low productivity. Our results provide important clues signaling imminent talik onset and C source transition, including (1) late cold-season (January–February) soil warming at depth ( ∼  2 m), (2) increasing cold-season emissions (November–April), and (3) enhanced respiration of deep, old C in warm permafrost and young, shallow C in organic-rich cold permafrost soils. Our results suggest a mosaic of processes that govern carbon source-to-sink transitions at high latitudes and emphasize the urgency of monitoring soil thermal profiles, organic C age and content, cold-season CO2 emissions, and atmospheric 14CO2 as key indicators of the permafrost C feedback.
Citation: Parazoo, N. C., Koven, C. D., Lawrence, D. M., Romanovsky, V., and Miller, C. E., Detecting the permafrost carbon feedback: Talik formation and increased cold-season respiration as precursors to sink-to-source transitions, The Cryosphere, 12, 123-144,, 2018.

Friday, January 12, 2018

Joe Romm: Stunning NASA chart shows how fast the ground beneath our feet is heating up

The land is warming twice as fast as the oceans … too bad we live on the land

by Joe Romm, Climate Progress, August 22, 2017

Global temperatures are rising faster on the land, where we live, than the oceans, where we don’t, NASA charts reveal. Since scientists have long predicted this trend and say it will continue, it’s worth a closer look.
Let’s start with the long-term global warming trend. According to NOAA, “Since 1880, surface temperature has risen at an average pace of 0.13 °F (0.07 °C) every 10 years, for a net warming of 1.71 °F (0.95 °C).”
But the warming is not evenly distributed: “Over this 136-year period, average temperature over land areas has warmed faster than ocean temperatures: 0.18 °F (0.10 °C) per decade compared to 0.11 °F (0.06 °C) per decade.” So over the entire record, the land is warming nearly 70 percent faster than the oceans.
But the warming is also speeding up. Over the last 45 years, surface temperature has been rising at an average rate of around 0.3 °F per decade — more than double the rate over the whole 135-year period. This speed up was also predicted. After all, emissions of CO2, the most important heat-trapping greenhouse gas, have increased by a factor of six since 1950 — and the rise of overall CO2 levels has sped up.
The disparity between the rate of land and ocean warming has also gotten bigger.  NASA Goddard Institute for Space Studies (GISS) recently posted some charts that show just how much faster it has been warming in recent decades — and how much the  disparity has grown.
In the past six decades, land temperatures have risen about  2.3 °F, a warming rate of nearly 0.4 °F a decade, as the top chart shows. That’s nearly double the temperature rise of the ocean, which is 1.25 °F per decade. Moreover, in the past 30 years, the rate of warming appears to have sped up even more, with land temperatures rising more than 0.6 °F a decade. That’s now a bit more than double the ocean warming.
But the key point, of course is that we live on the land. So when you see a rate of global warming quoted, remember, the rate of warming where we live is much higher — and growing fast.
Finally, you may be wondering why temperatures over the land are warming so much faster than temperatures over the ocean. Part of the reason is that the heat capacity of the ocean is so much greater than that of the land so its initial temperature response to warming is slower. As one explainer put it, “Think of the hot sand and cool water at the beach in the summer.” This is also why the ocean stores more than 90% of all of the excess heat from global warming.
Part of the reason the ocean warms more slowly is that much of the heating of the ocean goes into evaporation. But the land, particularly the drier parts of the planet, don’t have much moisture to evaporate  so much more of the global warming goes directly into temperature rise. For those technically minded readers who want a fuller explanation, start with this 2009 study, “Understanding Land–Sea Warming Contrast in Response to Increasing Greenhouse Gases.” Then try this 2013 study.

Three-quarters of the total insect population lost in protected nature reserves

from Radboud University Nijmegen, October 18, 2017

Since 1989, in 63 nature reserves in Germany the total biomass of flying insects has decreased by more than 75%. This decrease has long been suspected but has turned out to be more severe than previously thought. Ecologists from Radboud University together with German and English colleagues published these findings in the scientific journal PLoS ONE on October 18th, 2017.
In recent years, it became clear that the numbers of many types of insects such as butterflies and bees were declining in Western Europe and North America. 'However, the fact that flying insects are decreasing at such a high rate in such a large area is an even more alarming discovery,' states project leader at the Radboud University Hans de Kroon.
Thorough research
Entomologists (insect researchers) in Krefeld, Germany, led by Martin Sorg and Heinz Schwan, collected data over the past 27 years in 63 different places within nature reserves across Germany. Flying insects were trapped in malaise traps and the total biomass was then weighed and compared. The researchers from Nijmegen, Germany and England have now been able to analyse this treasure trove of data for the first time.
Decline also recorded in well-protected areas
The researchers discovered an average decline of 76 percent in the total insect mass. In the middle of summer, when insect numbers peak, the decline was even more severe at 82%. According to Caspar Hallmann from Radboud University who performed the statistical analyses, 'All these areas are protected and most of them are managed nature reserves. Yet, this dramatic decline has occurred.'
The exact causes of the decline are still unclear. Changes in the weather, landscape and plant variety in these areas are unable to explain this. The weather might explain many of the fluctuations within the season and between the years, but it doesn't explain the rapid downward trend.
A decline in other parts of the world too
Researchers can only speculate about the possible causes. 'The research areas are mostly small and enclosed by agricultural areas. These surrounding areas inflict flying insects and they cannot survive there. It is possible that these areas act as an 'ecological trap' and jeopardize the populations in the nature reserves,' explains Hallmann. It is likely that the results are representative for large parts of Europe and other parts of the world where nature reserves are enclosed by a mostly intensively used agricultural landscape.
Wake-up call
'As entire ecosystems are dependent on insects for food and as pollinators, it places the decline of insect eating birds and mammals in a new context,' states Hans de Kroon. 'We can barely imagine what would happen if this downward trend continues unabated.'
Because the causes of the decline are not yet known, it is difficult to take any concrete measures. The researchers hope that these findings will be seen as a wake-up call and prompt more research into the causes and support for long-term monitoring.
De Kroon: 'The only thing we can do right now is to maintain the utmost caution. We need to do less of the things that we know have a negative impact, such as the use of pesticides and prevent the disappearance of farmland borders full of flowers. But we also have to work hard at extending our nature reserves and decreasing the ratio of reserves that border agricultural areas.'

Shocking > 75% decline in flying insects in last 27 years

Since 1989, numbers of flying insects have dropped by more than 75% in parts of Germany, with serious implications for the rest of Europe.

by Tim Radford, Climate News Network, October 20, 2017

LONDON – The mass of flying insects in parts of Germany has fallen by three-quarters in the last 27 years. Since the territories sampled were all nature reserves in some way protected from pesticides and other disturbance, the implications are alarming: winged insects may be flying to oblivion across much of Europe.
The cost to natural ecosystems and to human economies could be devastating. Insects pollinate 80% of wild plants, feed on species that could otherwise become pests, recycle plant and animal waste, and are themselves food for 60% of birds. One calculation places the value of wild insect pollination at $57bn a year in the United States.

Vanishing insects

Researchers have already expressed concern about the vanishing numbers of butterflies in parts of Europe, possibly as a consequence of climate change. But the latest study does not distinguish individual species or even groups. It concentrates just on the sheer mass of flying insects in a German growing season.
The research – published in the Public Library of Science journal, PLOS ONE – assembles 1,503 records of winged insects, all caught in a standard field trap, in 63 unique locations in protected areas in lowland Germany during spring, summer and early autumn from 1989 to 2016. The data told a disconcerting story: the average seasonal mass of flying insects declined by 76% in under three decades. At the height of summer, the decline reached 82%.

“We need to do less of the things that we know
have a negative impact, such as the use of
pesticides, and prevent the disappearance
of farmland borders full of flowers”

The decline was consistent regardless of the type of habitat – dunes, heath land, rich and poor grasslands, wastelands, shrub cover and so on – and changes of land use or weather, or shifts in the habitat itself offered no obvious explanation. Researchers have identified reasons that one species, or a group of insects, might be at risk from climate change, perhaps because earlier flowering disrupts the feeding cycle or because the mix of species in an ecosystem changes with rising temperatures.
But there has always been an unspoken assumption that other species or groups of species may be likely to benefit from the change, by extending their range. The study is based on observations made only in one country. However, the finding implies that ecosystems across the whole of Europe could be affected, on a huge scale and at every level.

Downward trend

“As entire ecosystems are dependent on insects for food and as pollinators, it places the decline of insect-eating birds and mammals in a new context. We can barely imagine what would happen if this downward trend continues unabated,” says Hans de Kroon, an ecologist at Radboud University in Nijmegen in the Netherlands, one of the authors.
The only thing we can do right now is to maintain the utmost caution. We need to do less of the things that we know have a negative impact, such as the use of pesticides, and prevent the disappearance of farmland borders full of flowers. But we also have to work hard at extending our nature reserves and decreasing the ratio of reserves that border agricultural areas.”

Thursday, January 4, 2018

Climate risk: going mainstream

The governor of the Bank of England and ExxonMobil shareholders are just some of those changing the narrative on climate risk, says Dylan Tanner
Once climate change becomes a defining issue for financial stability, it may already be too late
by Dylan Tanner, The Actuary, September 7, 2017
In June this year, the Financial Stability Board’s (FSB’s)Task Force on Climate-related Financial Disclosures (TCFD) published its recommendations on how the corporate sector should disclose climate risk to investors. The FSB apparently regards climate change as a systemic financial risk, as articulated in a speech by the governor of the Bank of England, Mark Carney, in 2015. Meanwhile, at ExxonMobil’s annual general meeting in May this year, a majority of shareholders demanded that the oil and gas giant discloses its thinking on climate risk more clearly.
Data, analysis and advice on climate risk to portfolios have been around and available to investors for at least 20 years. By the late 1990s, the UN Environment Program (UNEP) Financial Initiative was messaging regularly on the risk of climate-change-induced weather events to the insurance sector and hence the wider markets.
In 2000, the investor-enabled Climate Disclosure Project (CDP) began collecting and aggregating carbon emissions information from thousands of companies around the globe. Financial data sets such as MSCI, Thomson Reuters Eikon and Bloomberg create and sell climate-related metrics on companies as part of their environment, social and governance (ESG) offering.
These observations beg two questions. Is climate risk now going mainstream in portfolio assessment? If so, what has changed? 
The answer to the first question is almost certainly yes, given the mainstream remit of the FSB and the universal nature of ExxonMobil’s shareholder base. The answer to the second is more complex. A key reason that climate risk analysis has not been widely accepted by financial analysts until now is that the messengers have largely come from the climate change and sustainability communities. The UNEP’s remit is to solve environmental issues, not advise on financial risk. Data sets such as CDP and the ESG metrics are regarded as originating in the sustainability agenda, and are used by investors mostly to satisfy sustainable investment commitments rather than inform mainstream risk strategy.
A more significant reason is captured by Carney in his 2015 ‘Tragedy of the Horizons’ speech, where he notes the disconnect between time horizons for current financial risk assessment and manifestation of the effects of climate change.

Predictions come to pass

Two decades have passed since the CDP and UNEP initiatives began, and some of the early indicators of these risks are now appearing. One of these relates to the fossil-fuel production sector, where groups such as the Carbon Tracker Initiative have predicted that a variety of climate-induced pressures could threaten the value of the reserves of oil, gas and coal on balance sheets. In November 2016, Shell shocked the market by estimating that oil demand could peak in as little as 5 years, “driven by efficiency and substitution,” according to then chief financial officer Simon Henry.
A 2016 report from think tank InfluenceMap showed the disparity between the predictions of global electric vehicle (EV) proliferation by the oil companies and those of the automakers and regulators. Toyota predicts 100% EVs and hybrids by 2050 in its sales. France pledges to ban petroleum-powered cars by 2040, and India has a goal of selling only EVs by 2030. Yet, the report notes, ExxonMobil forecasts that EVs will account for “less than 10% of new-car sales globally in 2040.” For a company that probably derives more than 30% of its revenues from petroleum-related transport, this disconnect is a concern. Shareholders are correct to demand further disclosure on the climate risk scenarios it is working with.
Other sectors on investors’ radar when it comes to climate risk and its disclosure include utilities. Reputational, financial and regulatory pressure on the use of coal for power generation is growing, while incentives for the scale-up of renewables is similarly accelerating. Bloomberg New Energy Finance estimates new power generation capacity will be mostly solar and wind by 2040, leaving gas, and especially coal generation and related value chains, as niche businesses. The power sector is one with long-term horizons and multi-decade plant life cycles, so understanding management strategy on future scenarios is essential for investors.

Funds flex their muscle

Pension funds are an important part of the global financial system, with the top 6,000 funds holding around $26trn (£20trn) of capital market assets. They have the ability to create market trends, and account for a significant portion of revenue generated by the financial sector as a whole. 
One of the largest such funds is Norway’s Government Pension Fund Global, with close to $1trn in assets. It adopted criteria in late 2015, allowing it to “exclude companies whose conduct to an unacceptable degree entails greenhouse gas emissions.” In June this year, the smaller but still substantial AP7 pension fund of Sweden announced it was divesting from ExxonMobil and five other companies for violation of the Paris climate agreement. Many other such funds may follow this trend. Such divestment and exclusion actions may be the last resort in an engagement chain, or intended as a signal on acceptable corporate governance. In the case of the Norwegian fund, its managers have a direct remit from the country’s parliament to consider global climate change risk in its management.

Change in data needs

While climate risk is now mainstream, the data needs of the investment community have shifted. For one thing, they are now highly sector-specific. Certain industries, such as energy and power generation and energy-intensive sectors like cement, are the focus, and investors want to understand management thinking on climate issues. To this end, the FSB recommendations stress disclosure by companies on the “resilience of an organisation’s strategy under climate-related scenarios, including a 2 °C or lower scenario” and the regulatory, market, technology and other changes these will bring.
Crucially, the FSB also extends its recommendations to the financial sector, and urges asset owners to test the resilience of the portfolio under the same scenarios. This approach necessitates a focus on forward-looking corporate behavioural metrics and analysis, as well as the carbon emissions accounting approach. For example, investors need to understand the capital asset allocation strategy of an electricity utility, and how this relates to regulatory trends. Likewise, they need to understand whether an oil/gas company’s business model is based on expecting to continue to be able to suppress climate-motivated regulations, and likely scenarios should the political climate shift suddenly.

Mainstream methods apply

Disclosures in line with the TCFD’s recommendations do not feed into any legally binding financial disclosure processes, such as those of the U.S. Securities And Exchange Commission. As a result, achieving universal participation – especially by the most at-risk companies –remains a challenge. 
The TCFD and other disclosure systems aside, mainstream analysis of corporations by investors involves reliance on other sources of information, such as discussions with senior management and third-party investigations.
In the climate risk context, this process will spur the financial research and data sectors to acquire expertise, and perhaps to form unusual alliances with climate specialists in the NGO, academic and technology sectors, to better understand the nuances of portfolio, sector and company risk.

Dylan Tanner is executive director at InfluenceMap

Michael Mann: A 'Perfect' Storm: Extreme Winter Weather, Bitter Cold, and Climate Change

by Michael Mann, The Climate Reality Project, January 4, 2018
The US East Coast is experiencing an “old-fashioned” winter, with plenty of cold weather and some heavy snowfall in certain places. Listening to climate contrarians like President Donald Trump, you might think this constitutes the death knell for concern over human-caused climate change.
Yet, what we were witnessing play out is in fact very much consistent with our expectations of the response of weather dynamics to human-caused climate change.
Let’s start with the record five-plus feet of snowfall accumulation in Erie, Pennsylvania, in late December. Does this disprove global warming? “Exactly the opposite,” explains my colleague, Dr. Katharine Hayhoe of Texas Tech University. 
Global warming is leading to later freeze-up of the Great Lakes and warmer lake temperatures. It is the collision of cold Arctic air with relatively warm unfrozen lake water in early winter that causes lake effect snows in the first place. The warmer those lake temperatures, the more moisture in the air, and the greater potential for lake effect snows. Not surprisingly, we see a long-term increase in lake effect snowfalls as temperatures have warmed during the last century (see figure below).
iew image on Twitter
How about those frigid low temperatures back east this winter? Surely that extreme cold must disprove global warming?
Once again, the claim is misguided. While we have seen some daily all-time lows for a smattering of locations in the US, these pale in comparison with the number of all-time highs we’ve seen over the past year. In fact, the record highs have outpaced the record lows 61 to seven, i.e. nine times more often (see table below), consistent with what we expect to see as the globe continues to warm.
Moreover, while we’ve seen some cold weather in the eastern half of the North America (see the pattern for New Year’s Day below), the western half of North America has been unusually warm. Indeed, most of the Northern Hemisphere, and the globe overall, have been unusually warm. That’s why we call it global warming, folks.
(Image obtained using Climate Reanalyzer, Climate Change Institute, University of Maine, USA)
But what about this pattern of cold in the eastern US and warm in the western US? This so-called “dipole” pattern has become more common in recent winters, and recent research suggests that climate change may be favoring this contrast in temperature by causing the jet stream to meander in a particular pattern, with an upward meander or “ridge” in the west bringing warm air up from the south and a downward meander or “trough” in the east, bringing cold air down from the north. Some scientists think that the dramatic loss of sea ice in the Arctic may be favoring this jet stream pattern.
Finally, the news is abuzz today with an impending “massive Nor’easter,” a “bomb cyclone” that is “set to explode” in the days ahead (see plot below). This isn’t just hype. The National Weather Service has warned that “this rapidly intensifying East Coast storm will produce strong, damaging winds — possibly resulting in downed trees, power outages, and coastal flooding.”
With a central pressure forecast to drop very low (see plot below), the storm will threaten the record set by unprecedented 2012 Superstorm Sandy as the lowest surface pressure ever measured in the North Atlantic north of Cape Hatteras (the central surface pressure of a storm is one measure of its strength).
(© 2018 ECMWF cc by nc nd 4.0)
Surely such a massive winter storm, with its promise of bitter cold winds and potentially heavy coastal snowfalls, must be evidence against the climate crisis?
Once again, rather the opposite is true. East Coast winter storms, known as “nor’easters” because of the unusual northeasterly direction of the winds as the storm spirals in from the south, are unusual in that they derive their energy not just from large contrasts in temperature that drive most extratropical storm systems, but also from the energy released when water evaporates from the (relatively warm) ocean surface into the atmosphere.
This is a characteristic that these storms share with tropical storms and hurricanes. The warmer the ocean surface, the more energy that is available to intensify these storms. And the warmer the ocean surface, the more moisture there is in the atmosphere – moisture that is available to form precipitation. As the winds wrap around in a counter-clockwise manner, they bring all of that moisture northwest, where it is chilled and ultimately falls not as rain but snow. Lots of snow.
As the oceans continue to warm, cold Arctic air masses collide with increasingly warm Atlantic Ocean waters. That means larger temperature contrasts and potentially stronger storms. But those warmer oceans also mean more moisture in the atmosphere, even more energy to strengthen the storm, and the potential for larger snowfalls.  We might, if you’ll forgive the pun, call this a “perfect storm” of factors for intensification.
Indeed, climate model simulations indicate that we can expect more intense nor’easters as human-caused climate change continues to warm the oceans.
(Image obtained using Climate Reanalyzer, Climate Change Institute, University of Maine, USA)
This leads us back to the current strengthening storm. The entire North Atlantic is unusually warm right now (+0.6 degrees Celsius) relative to the already-globally-warmed, late-twentieth-century average (1971-2000), and there are large patches of ocean water off the US East Coast that are 2-4 degrees Celsius above that average. The storm will be encountering that exceptional ocean heat as it travels northward along the US coastline, and that is part of why it has a very good chance of becoming the most intense nor’easter we’ve yet observed.
So, to the climate change doubters and deniers out there, the unusual weather we’re seeing this winter is in no way evidence against climate change. It is an example of precisely the sort of extreme winter weather we expect because of climate change.
Stay up to date with the latest in climate fight and insight from influential scientists and voices like Dr. Mann by signing up for our activist email list today.
Dr. Michael Mann is distinguished professor of atmospheric science at Penn State University and author of The Hockey Stick and The Climate Wars and, more recently, The Madhouse Effect.

"Increased rainfall volume from future convective storms in the US" by Andreas Prein et al., Nature Climate Change, 7 (2017); doi: 10.1038/s41558-017-0007-7

Nature Climate Change, 7 (2017) 880884; doi: 10.1038/s41558-017-0007-7

Increased rainfall volume from future convective storms in the US

Mesoscale convective system (MCS)-organized convective storms with a size of ~100 km have increased in frequency and intensity in the USA over the past 35 years1, causing fatalities and economic losses2. However, their poor representation in traditional climate models hampers the understanding of their change in the future3. Here, a North American-scale convection-permitting model which is able to realistically simulate MSCs4 is used to investigate their change by the end-of-century under RCP8.5 (ref. 5). A storm-tracking algorithm6 indicates that intense summertime MCS frequency will more than triple in North America. Furthermore, the combined effect of a 15–40% increase in maximum precipitation rates and a significant spreading of regions impacted by heavy precipitation results in up to 80% increases in the total MCS precipitation volume, focused in a 40-km radius around the storm center. These typically neglected increases substantially raise future flood risk. Current investments in long-lived infrastructures, such as flood protection and water management systems, need to take these changes into account to improve climate-adaptation practices.

North American Storm Clusters Could Produce 80 Percent More Rain Say NCAR Scientists

by Floodlist News, November 28, 2017

Major clusters of summertime thunderstorms in North America will grow larger, more intense, and more frequent later this century in a changing climate, unleashing far more rain and posing a greater threat of flooding across wide areas, new research concludes.

The study, by scientists at the National Center for Atmospheric Research (NCAR), builds on previous work showing that storms are becoming more intense as the atmosphere is warming. In addition to higher rainfall rates, the new research finds that the volume of rainfall from damaging storms known as mesoscale convective systems (MCSs) will increase by as much as 80% across the continent by the end of this century, deluging entire metropolitan areas or sizable portions of states.

“The combination of more intense rainfall and the spreading of heavy rainfall over larger areas means that we will face a higher flood risk than previously predicted,” said NCAR scientist Andreas Prein, the study’s lead author. “If a whole catchment area gets hammered by high rain rates, that creates a much more serious situation than a thunderstorm dropping intense rain over parts of the catchment.”
“This implies that the flood guidelines which are used in planning and building infrastructure are probably too conservative,” he added.
The research team drew on extensive computer modeling that realistically simulates MCSs and thunderstorms across North America to examine what will happen if emissions of greenhouse gases continue unabated.
The study will be published November 20, 2017, in the journal Nature Climate Change. It was funded by the National Science Foundation, which is NCAR’s sponsor, and by the U.S. Army Corps of Engineers.

Hourly rain rate averages for the 40 most extreme summertime mesoscale convective systems (MCSs) in the current (left) and future climate of the mid-Atlantic region. New research shows that MSCs will generate substantially higher maximum rain rates over larger areas by the end of the century if society continues a “business as usual” approach of emitting greenhouse gases . Image: ©UCAR, Image by Andreas Prein, NCAR.

This satellite image loop shows an MCS developing over West Virginia on June 23, 2016. The resulting floods caused widespread flooding, killing more than 20 people. MCSs are responsible for much of the major flooding east of the Continental Divide during warm weather months. (Image by NOAA National Weather Service, Aviation Weather Center.)

A Warning Signal

Thunderstorms and other heavy rainfall events are estimated to cause more than $20 billion of economic losses annually in the United States, the study notes. Particularly damaging, and often deadly, are MSCs: clusters of thunderstorms that can extend for many dozens of miles and last for hours, producing flash floods, debris flows, landslides, high winds, and/or hail. The persistent storms over Houston in the wake of Hurricane Harvey were an example of an unusually powerful and long-lived MCS.
Storms have become more intense in recent decades, and a number of scientific studies have shown that this trend is likely to continue as temperatures continue to warm. The reason, in large part, is that the atmosphere can hold more water as it gets warmer, thereby generating heavier rain.
A study by Prein and co-authors last year used high-resolution computer simulations of current and future weather, finding that the number of summertime storms that produce extreme downpours could increase by five times across parts of the United States by the end of the century. In the new study, Prein and his co-authors focused on MCSs, which are responsible for much of the major summertime flooding east of the Continental Divide. They investigated not only how their rainfall intensity will change in future climates, but also how their size, movement, and rainfall volume may evolve.
Analyzing the same dataset of computer simulations and applying a special storm-tracking algorithm, they found that the number of severe MCSs in North America more than tripled by the end of the century. Moreover, maximum rainfall rates became 15-40% heavier, and intense rainfall reached farther from the storm’s center. As a result, severe MCSs increased throughout North America, particularly in the northeastern and mid-Atlantic states, as well as parts of Canada, where they are currently uncommon.
The research team also looked at the potential effect of particularly powerful MCSs on the densely populated Eastern Seaboard. They found, for example, that at the end of the century, intense MCSs over an area the size of New York City could drop 60% more rain than a severe present-day system. That amount is equivalent to adding six times the annual discharge of the Hudson River on top of a current extreme MCS in that area.
“This is a warning signal that says the floods of the future are likely to be much greater than what our current infrastructure is designed for,” Prein said. “If you have a slow-moving storm system that aligns over a densely populated area, the result can be devastating, as could be seen in the impact of Hurricane Harvey on Houston.”

Intensive Modeling

Advances in computer modeling and more powerful supercomputing facilities are enabling climate scientists to begin examining the potential influence of a changing climate on convective storms such as thunderstorms, building on previous studies that looked more generally at regional precipitation trends.
For the new study, Prein and his co-authors turned to a dataset created by running the NCAR-based Weather and Research Forecasting (WRF) model over North America at a resolution of 4 kilometers (about 2.5 miles). That is sufficiently fine-scale resolution to simulate MCSs. The intensive modeling, by NCAR scientists and study co-authors Roy Rasmussen, Changhai Liu, and Kyoko Ikeda, required a year to run on the Yellowstone system at the NCAR-Wyoming Supercomputing Center.
The team used an algorithm developed at NCAR to identify and track simulated MCSs. They compared simulations of the storms at the beginning of the century, from 2000 to 2013, with observations of actual MCSs during the same period, and showed that the modeled storms are statistically identical to real MCSs.
The scientists then used the dataset and algorithm to examine how MCSs may change by the end of the century in a climate that is approximately 5 degrees Celsius (9 degrees Fahrenheit) warmer than in the pre-industrial era — the temperature increase expected if greenhouse gas emissions continue unabated.

About the paper

Title: Increased rainfall volume from future convective storms in the US
Authors: Andreas F Prein, Changhai Liu, Kyoko Ikeda, Stanley B Trier, Roy M Rasmussen, Greg J Holland, Martyn P Clark
Source: University Corporation for Atmospheric Research