A controversial paper published two years ago that concluded there was no detectable slowdown in ocean warming over the previous 15 years — widely known as the “global warming hiatus” — has now been confirmed using independent data in research led by researchers from UC Berkeley and Berkeley Earth, a non-profit research institute focused on climate change.
A NEMO float, part of the global Argo array of ocean sensing stations, deployed in the Arctic from the German icebreaker Polarstern Bremerhaven. (Photo courtesy of Argo)
The 2015 analysis showed that the modern buoys now used to measure ocean temperatures tend to report slightly cooler temperatures than older ship-based systems, even when measuring the same part of the ocean at the same time. As buoy measurements have replaced ship measurements, this had hidden some of the real-world warming.
After correcting for this “cold bias,” researchers with the National Oceanic and Atmospheric Administration concluded in the journal Science that the oceans have actually warmed 0.12 degrees Celsius (0.22 degrees Fahrenheit) per decade since 2000, nearly twice as fast as earlier estimates of 0.07 degrees Celsius per decade. This brought the rate of ocean temperature rise in line with estimates for the previous 30 years, between 1970 and 1999.
This eliminated much of the global warming hiatus, an apparent slowdown in rising surface temperatures between 1998 and 2012. Many scientists, including the International Panel on Climate Change, acknowledged the puzzling hiatus, while those dubious about global warming pointed to it as evidence that climate change is a hoax.
Climate change skeptics attacked the NOAA researchers and a House of Representatives committee subpoenaed the scientists' emails. NOAA agreed to provide data and respond to any scientific questions but refused to comply with the subpoena, a decision supported by scientists who feared the “chilling effect” of political inquisitions.
The new study, which uses independent data from satellites and robotic floats as well as buoys, concludes that the NOAA results were correct. The paper will be published Jan. 4 in the online, open-access journal Science Advances.
“Our results mean that essentially NOAA got it right, that they were not cooking the books,” said lead author Zeke Hausfather, a graduate student in UC Berkeley's Energy and Resources Group.
Long-term climate records
Hausfather said that years ago, mariners measured the ocean temperature by scooping up a bucket of water from the ocean and sticking a thermometer in it. In the 1950s, however, ships began to automatically measure water piped through the engine room, which typically is warm. Nowadays, buoys cover much of the ocean and that data is beginning to supplant ship data. But the buoys report slightly cooler temperatures because they measure water directly from the ocean instead of after a trip through a warm engine room.
A new UC Berkeley analysis of ocean buoy (green) and satellite data (orange) show that ocean temperatures have increased steadily since 1999, as NOAA concluded in 2015 (red) after adjusting for a cold bias in buoy temperature measurements. NOAA's earlier assessment (blue) underestimated sea surface temperature changes, falsely suggesting a hiatus in global warming. The lines show the general upward trend in ocean temperatures. (Zeke Hausfather graphic)
NOAA is one of three organizations that keep historical records of ocean temperatures – some going back to the 1850s – widely used by climate modelers. The agency's paper was an attempt to accurately combine the old ship measurements and the newer buoy data.
Hausfather and colleague Kevin Cowtan of the University of York in the UK extended that study to include the newer satellite and Argo float data in addition to the buoy data.
“Only a small fraction of the ocean measurement data is being used by climate monitoring groups, and they are trying to smush together data from different instruments, which leads to a lot of judgment calls about how you weight one versus the other, and how you adjust for the transition from one to another,” Hausfather said. “So we said, ‘What if we create a temperature record just from the buoys, or just from the satellites, or just from the Argo floats, so there is no mixing and matching of instruments?'”
In each case, using data from only one instrument type – either satellites, buoys or Argo floats – the results matched those of the NOAA group, supporting the case that the oceans warmed 0.12 degrees Celsius per decade over the past two decades, nearly twice the previous estimate. In other words, the upward trend seen in the last half of the 20th century continued through the first 15 years of the 21st: there was no hiatus.
“In the grand scheme of things, the main implication of our study is on the hiatus, which many people have focused on, claiming that global warming has slowed greatly or even stopped,” Hausfather said. “Based on our analysis, a good portion of that apparent slowdown in warming was due to biases in the ship records.”
Correcting other biases in ship records
In the same publication last year, NOAA scientists also accounted for changing shipping routes and measurement techniques. Their correction – giving greater weight to buoy measurements than to ship measurements in warming calculations – is also valid, Hausfather said, and a good way to correct for this second bias, short of throwing out the ship data altogether and relying only on buoys.
Berkeley's analysis of ocean buoy (green) and satellite data (orange) and NOAA's 2015 adjustment (red) are compared to the Hadley data (purple), which have not been adjusted to account for some sources of cold bias. The Hadley data still underestimate sea surface temperature changes. (Zeke Hausfather graphic)
Another repository of ocean temperature data, the Hadley Climatic Research Unit in the United Kingdom, corrected their data for the switch from ships to buoys, but not for this second factor, which means that the Hadley data produce a slightly lower rate of warming than do the NOAA data or the new UC Berkeley study.
“In the last seven years or so, you have buoys warming faster than ships are, independently of the ship offset, which produces a significant cool bias in the Hadley record,” Hausfather said. The new study, he said, argues that the Hadley center should introduce another correction to its data.
“People don't get much credit for doing studies that replicate or independently validate other people's work. But, particularly when things become so political, we feel it is really important to show that, if you look at all these other records, it seems these researchers did a good job with their corrections,” Hausfather said.
Co-author Mark Richardson of NASA‘s Jet Propulsion Laboratory and the California Institute of Technology in Pasadena added, “Satellites and automated floats are completely independent witnesses of recent ocean warming, and their testimony matches the NOAA results. It looks like the NOAA researchers were right all along.“
Other co-authors of the paper are David C. Clarke, an independent researcher from Montreal, Canada, Peter Jacobs of George Mason University in Fairfax, Virginia, and Robert Rohde of Berkeley Earth. The research was funded by Berkeley Earth.
BERKELEY - The first of three large land donations from Pacific Gas and Electric Company (PG&E) to the University of California has been officially transferred, expanding UC's research forest lands by 1,459 acres.
Named the “Grouse Ridge Forest” after the dominant feature of the property, the land is located on three parcels in the headwaters of the Yuba River in Nevada County. In conjunction with the land donation to UC, a conservation easement was conveyed to the Bear Yuba Land Trust (BYLT), ensuring the permanent protection of the forest land and important wildlife habitat there.
“As California's forests experience increased stresses from droughts, beetles, fires, and climate change, we need more “living laboratories” to learn how we can increase the resiliency of these critical watersheds over the next century,” said Bill Stewart, forestry specialist and co-director of the UC Berkeley Center for Forestry (CFF). “This new addition of research forest land is valuable as another site along a north-south transect of the Sierra Nevada that ensures that research results are broadly applicable and not just valid in one specific location.”
The University of California now has 6,452 acres of research forests, which are managed by the Center for Forestry. Through research, education and public service, the CFF continues to improve scientific understanding of the interconnected role of California's forests and state watersheds, renewable wood products, fish and wildlife habitat, scenic and recreational opportunities, and climate benefits.
These new lands will allow for increased research on the effects of climate change on forest ecosystems, expanded experimentation of forest-management techniques, and broadened outreach efforts to students of all levels, researchers, and the interested public.
“The importance of research forests as a space for studies on both mitigation of and adaptation to climate change was highlighted again this week with the announcement that we had just had the hottest year on record, for the third year in a row,” said J. Keith Gilless, CNR dean and professor of forest economics.
This is the first time UC has owned a forest property while another entity holds the conservation easement. “The Land Trust is excited for the opportunity to be a partner with the University of California in this endeavor,” said BYLT Executive Director Marty Coleman Hunt in an announcement made by the organization in December. “The forest has been a habitat for wildlife like mountain lion, deer and coyote and will remain so for as long as the forest can support them. As the forest changes over time, the University of California will study how nature adapts, and how the impact of humans can harm or benefit the natural processes.”
The land donation became official with the close of escrow in December 2016. It was originally approved by the Pacific Forest and Watershed Lands Stewardship Council in 2004 as part of PG&E's bankruptcy settlement, with the goal of ensuring that over 140,000 acres of California's lakes and watershed lands are conserved for the public good and to serve California's young people.
Two more forests have also been pledged to UC by PG&E and are expected to be officially transferred over the next few years: one along Marble Creek in eastern Shasta County and another near along the Bear River that is the dividing line between Placer County and Nevada County.
Once complete, these three donations will more than double the number of acres of UC research forest lands.
Using climate data and aerial tree mortality surveys conducted by the U.S. Forest Service during four years (2012-2015) of extreme drought in California, they found that when a drought hits the region, trees growing in areas that are already dry are most susceptible.
The research also showed that the effects of drought on forests can take years to surface, suggesting that such effects may linger even after the drought has ended.
Southern Sierra Nevada trees are most vulnerable
The study said that trees in the driest and densest forests are the most at risk of dying in an extreme drought. In California, that makes crowded stands of trees in the Southern Sierra Nevada the most vulnerable in the state.
The concept is simple: Trees in dense forests are like multiple straws competing for the same glass of water. In wet climate conditions, that competition goes largely unnoticed. But when it's dry, few are able to quench their thirst, setting the stage for mass mortality.
‘How much drought a tree can take'
“Our analysis found out how much drought a tree can take,” said UC Davis Ph.D. student Derek Young, who co-led the study with Jens Stevens, a UC Davis postdoctoral researcher during the study who is currently at UC Berkeley, and Mason Earles, a postdoctoral researcher at Yale University. “If forest managers want to get the biggest bang for their buck in reducing forest vulnerability to drought, this study suggests they should focus on the densest stands in the driest areas. And when we reestablish forests burned by severe wildfire in these areas, we should plant at lower densities from the beginning.”
Tree mortality in the Sierra Nevada in 2015 was the worst in recorded history. The U.S. Forest Service aerial tree mortality surveys in 2015 estimated 29 million trees in California had died after four years of extreme drought.
Though the drought began in 2012, major effects on trees did not appear immediately. While some trees died every year, mortality spiked only in the fourth year of extreme drought.
Tree mortality a delayed reaction
In a blog post he wrote on the subject in May 2016, Young noted: “This observation highlights the fact that tree mortality can take several years to respond to drought. Such a delayed response is often observed in studies of drought stress, and the existence of this delayed response hints that we are likely to observe high mortality well into 2016 and potentially beyond, especially in Southern California.”
Indeed, surveys conducted by the U.S. Forest Service in 2016 estimated an additional 62 million trees died that year.
Other study co-authors include UC Davis associate professor Andrew Latimer, and Jeffrey Moore, Adam Ellis and Amy Jirka with the USDA Forest Service.
Study authors were supported by a National Science Foundation Graduate Research Fellowship, EPA STAR Fellowship, and a USDA Hatch Project.
Research ecologist Rahel Sollmann receives UC President's Award to study fire impact on forests
The King Fire, which burned more than 97,000 acres in El Dorado County in 2014, will be studied by UC Davis ecologist Rahel Sollman and others. The other photo of Rahel, let's crop and use as an inset. (U.S. Forest Service)
RahelSollmann, an assistant professor in the UC Davis Department of Wildlife, Fish and Conservation Biology, is one of three recipients of the 2017 Research Catalyst Awards announced recently by University of CaliforniaPresidentJanetNapolitano.
With the $271,000 in funding provided by the award, Sollmann, a quantitative ecologist, will help analyze how megafires disrupt the forest ecosystem and food web and pollination networks. She will work with UC Berkeley Associate Professor Justin Brashares and UC Santa Barbara Assistant Professor Hillary Young, as well as other researchers at UC Davis and the U.S. Forest Service. Their research will focus on the King Fire, which burned more than 97,000 acres in El Dorado County in 2014.
“Forests are really at the heart of California economy, culture, and its ecological heritage,” Sollmann said. “The Forest Service has identified these large fires as the major threat to the integrity of California forests. We really hope to understand how large fires change forest pollination and food networks.”
Rahel Sollmann (John Stumbos/UC Davis)
Napolitano launched the President's Research Catalyst Awards in 2014tofostermulticampus, interdisciplinary research in areas of strategic importance to California and the world. Since its inception, the program has provided nearly $10 million for research focused on climate change, cultural preservation, drought, basic science, and other areas. It has created research opportunities for 50 UC faculty, nearly five dozen graduate students, and 20 undergraduates. The three awards, totaling more than $2 million, involve faculty and students from across the university with lead campuses at UC Santa Barbara, UC Santa Cruz, and UC Davis. The other projects include research led by UC Santa Barbara aimed at improving teacher training and student outcomes in California's K-12 schools ($1.5 million); and a UC Santa Cruz project ($278,000) to enhance knowledge of humanhealththroughpaleogenomic research.
Rahel Sollmann Recipients were selected following a highly competitive review process. A panel of experts evaluated applications based on scholarly merit and likely impact. President Napolitano selected the winners from among the top-ranked proposals representing all areas of university scholarship. Funding for the Catalyst Awards comes from the UC president's endowment funds, which support systemwide initiatives and projects.
Only about half of conifer trees regenerated five to seven years after wildfire in sites studied.
Study spanned 10 national forests and 4 burned areas in California.
Study presents tool to help foresters prioritize which lands to plant after a wildfire.
A study spanning 10 national forests and 14 burned areas in California found that conifer seedlings were found in less than 60 percent of the study areas five to seven years after fire. Of the nearly 1,500 plots surveyed, 43 percent showed no natural conifer regeneration at all.
The study was co-led by UC Davis and the USDA Forest Service and published December 21 in the journal Ecosphere. It presents a tool to help foresters prioritize which lands to replant immediately after a fire, and which lands they can expect to regrow naturally.
“High-severity fires are knocking out seed sources and leading to a natural regeneration bottleneck, which poses a predicament for the sustainability of our forests,” said lead author Kevin Welch, a research associate with the UC Davis Department of Plant Sciences.
For example, 10 of the 14 burned areas in the study, which include well-known wildfires like the Moonlight (2007) and Power (2009) fires, did not meet Forest Service stocking density thresholds for mixed conifer forests, making them good candidates for replanting and restoration efforts.
“Knowing that the Forest Service doesn't have the time, budget and staffing levels to restore everything, we basically want to help foresters predict what will be there five to seven years later so they can better focus restoration efforts,” Welch said.
How Does The Tool Work?
The researchers surveyed a range of elevations, forest types and fire severities –including in the Sierra Nevada, Klamath Mountains, and North Coast regions –to determine which factors promote and limit natural conifer regeneration and how different conifer species respond after a fire.
Using a simple tool developed by the research team, a manager can enter the forest the year following a fire and take a few field measurements –including distance to seed source, slope, and the cross-sectional area of living trees in the nearby forest. They can then predict whether a severely burned site is likely to meet a desired level of tree density five to seven years later.
Tested against four wildfires that were not in the study, the researchers found the tool was able to predict with more than 70 percent accuracy whether an area would likely need to be replanted or not.
The study plots were in California, but the authors suggest study results could apply to mixed conifer forests across the North American Mediterranean Climate Zone, which stretches from southwestern Oregon through California to northern Baja California and includes parts of western Nevada.
A Race For The Sun
As the research team saw while hiking through miles of dense brush, high-severity fires also stimulate shrub growth to the detriment of fire-resistant tree species that foresters try to encourage. The conifer regeneration that is occurring is heavily dominated by species that tolerate shade but not fire, such as Douglas fir, white fir and incense cedar.
Fire-resistant and drought-tolerant trees, such as ponderosa, sugar and Jeffrey pine, do not tolerate shade well. Such species are likely to better withstand the warmer, drier climates projected for California in the future.
Currently however, forest and fire conditions are not favorable for the survival of these more desirable trees. According to the study, strategies for increasing pines in California forests include reducing forest densities and fire severities while increasing overall fire occurrence (both prescribed fires and managed wildfires). They also suggest planting pines before shrubs and shade-tolerant trees crowd them out and remove their light source.
“As western forests increasingly experience warmer weather and more frequent and more severe fires, a better understanding of what conifers need to regenerate naturally after fire can help us create and manage more sustainable, resilient forests,” said co-author Hugh Safford, regional ecologist for the USDA-Forest Service's Pacific Southwest Region and a member of the adjunct faculty in the UC Davis Department of Environmental Science and Policy.
The study was funded by the USDA Forest Service and UC Davis.