Kai Zhu

Global Change Biology across Space and Time

Grassland paper chosen as cover in PNAS

Zhu K., Chiariello N.R., Tobeck T., Fukami T., Field C.B. (2016). Nonlinear, interacting responses to climate limit grassland production under global change. Proceedings of the National Academy of Sciences. 113, 10589-10594.

Paper link

Cover link

Cover image: Pictured is grassland at the Jasper Ridge Biological Preserve in California. Over 17 years, Kai Zhu et al. subjected this grassland to changes in temperature, precipitation, carbon dioxide, and nitrogen, both individually and in combination, to determine how simultaneous changes in multiple global environmental factors would affect primary production. Primary production was a ridge-shaped function of temperature and precipitation, with peak production increasing with added nitrogen and shifting to lower temperatures with added carbon dioxide. See the article by Zhu et al. on pages 10589–10594. Image courtesy of Dan Quinn (Stanford University, Stanford, CA).

Grassland paper published in PNAS

Zhu K., Chiariello N.R., Tobeck T., Fukami T., Field C.B. (2016). Nonlinear, interacting responses to climate limit grassland production under global change. Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1606734113.

Paper link

Rice University News link

Carnegie Institution News link

Inside Science — American Institute of Physics link

Associated Press link

Warmer, wetter climate would impair California grasslands

17-year experiment finds present climate near optimal for plant growth 

Jade Boyd/Rice University

HOUSTON — (Sep. 5, 2016) — Results from one of the longest-running and most extensive experiments to examine how climate change will affect agricultural productivity show that California grasslands will become less productive if the temperature or precipitation increases substantially above average conditions from the past 40 years.

Grasslands at Jasper Ridge Biological Preserve

Grassland at Stanford University’s Jasper Ridge Biological Preserve. An examination of 17 years of experimental data from the preserve is helping scientists from Rice University, Stanford and the Carnegie Institution for Science better understand how ecosystems will respond to climate change. (Photo courtesy of Daniel J. Quinn/Stanford University)

That’s one conclusion from a new study in this week’s Early Edition of the Proceedings of the National Academy of Sciences by researchers from Rice University, Stanford University and the Carnegie Institution for Science. The research team analyzed data from the Jasper Ridge Global Change Experiment, which has run continuously since 1998. The experiment simulates the effect of warmer temperatures, increased atmospheric carbon dioxide, increased nitrogen pollution and increased rainfall on a 1.8-acre tract at Stanford’s Jasper Ridge Biological Preserve.

“There’s been some hope that changing climate conditions would lead to increased productivity of grasses and other plants that draw down carbon dioxide from the atmosphere,” said study lead author Kai Zhu, a global ecologist and data scientist at Rice “In northern California, it was hypothesized that net grassland productivity might increase under the warmer, wetter conditions that are predicted by most long-term climate models. Our evidence disproves that idea.”

The Jasper Ridge experiment involves 136 test plots where scientists can study how grass will grow under conditions that are predicted to occur later this century due to climate change. The experiment allows scientists to test four variables: higher temperatures, increased precipitation, increased atmospheric CO2 levels and increased nitrogen levels. The plots are configured in such a way that scientists can test each of the variables independently and in combination.

Kai Zhu

Kai Zhu

Global change is quite complicated,” said Zhu, who spent almost two years analyzing Jasper Ridge data during a postdoctoral fellowship at Stanford and the Carnegie Institution for Science from 2014 to 2016. “It does not just mean change in temperature. There are also changes in rainfall, atmospheric CO2, nitrogen and many other things. If we want to get a comprehensive understanding of everything, it is important to have experiments like Jasper Ridge, which manipulate more than one variable, both singly and in combination.”

One clear finding from the data is that increased levels of CO2 did not increase grass production. Instead, the amount of grass grown at sites with elevated CO2 remained flat, even at CO2 levels almost twice the present atmospheric concentration.

“The nonresponse to CO2 is as important as any of our other findings,” Zhu said. “That finding may surprise people because a lot have said that if you have more CO2 in the atmosphere, you’ll get better growth because CO2 is a resource for plants. That’s a popular hypothesis.”

Researchers sampling grassland plots

Researchers sampling grassland plots at the Jasper Ridge Global Change Experiment. (Image courtesy of Nona Chiariello/Stanford University)

By examining data from all the test plots, including those where CO2 increased in conjunction with higher temperature, rainfall and nitrogen levels, and incorporating more than 40 years of climate records from the Jasper Ridge site, Zhu was able to deduce the optimal temperature and moisture levels for production under all conditions. His analysis showed that average conditions from the past 40 years are near optimal for grass production, and any significant deviation toward warmer or wetter conditions will cause the land to be less productive.

“Experiments like Jasper Ridge are designed to examine the interactive and unexpected effects that are likely to arise from global environmental change,” said study co-author Chris Field, the founding director of the Carnegie Institution’s Department of Global Ecology and the Melvin and Joan Lane Professor for Interdisciplinary Environmental Studies at Stanford University. “The nonlinear, interactive effects of temperature and precipitation on grassland primary production revealed by this analysis highlight the value of this experimental approach and suggest that it could be useful in studying how global change will affect other types of ecosystems.”

Additional co-authors include Nona Chiariello and Tadashi Fukami, both of Stanford, and Todd Tobeck of the Carnegie Institution. The research was supported by the National Science Foundation, the Department of Energy, the Packard Foundation, the Morgan Family Foundation, the Alexander von Humboldt Foundation, Stanford University and the Carnegie Institution for Science.

Coral reef paper published in Nature

Albright R., Hosfelt J., Kwiatkowski L., Maclaren J.K., Mason B.M., Nebuchina Y., Ninokawa A., Pongratz J., Ricke K.L., Rivlin T., Schneider K., Sesboüé M., Shamberger K., Silverman J., Wolfe K., Zhu K., Caldeira K. (2016). Reversal of ocean acidification enhances net coral reef calcification. Nature. DOI 10.1038/nature17155

News release below (Rice link here).

Study: Ocean acidification already slowing coral reef growth

Experiment dials back clock to test ocean reef growth in preindustrial conditions 

Jade Boyd/Rice University

HOUSTON — (Feb. 25, 2016) — An international team of scientists from the Carnegie Institution for Science, Rice University and other institutions has performed the first experiment to manipulate seawater chemistry in a natural coral-reef community to determine the effect that excess carbon dioxide released by human activity is having on coral reefs.

The research, which is published in this week’s issue of Nature, was conducted in a lagoon on the southern Great Barrier Reef in Australia in 2014. By controlling the alkalinity on a portion of the reef, the team was able to examine how fast the reef is growing today and compare that with growth rates in less acidic conditions that existed prior to the Industrial Revolution.

coral reef

By controlling the alkalinity on a portion of the Great Barrier Reef, a team that included Rice University BioSciences researcher Kai Zhu was able to examine how fast the reef is growing today and compare that with growth rates in less acidic conditions that existed prior to the Industrial Revolution.

“Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already causing reefs to grow more slowly than they did 100 years ago,” said study lead author Rebecca Albright, a marine biologist in Carnegie’s Department of Global Ecology in Stanford, Calif. “Ocean acidification is already taking its toll on coral reef communities. This is no longer a fear for the future; it is the reality of today.”

The research team included Rice’s Kai Zhu, an expert in ecological statistics who joined Rice as a Huxley Faculty Fellow in the Department of BioSciences in January following a postdoctoral appointment at Carnegie’s Department of Global Ecology.

Kai Zhu

Kai Zhu (Photo by Jeff Fitlow/Rice University)

“The data analysis for the experiment was complicated by the natural variation of conditions in the reef,” Zhu said. “Statistically speaking, there was a great deal of noise in the data, and as scientists we needed to filter out the noise so that we could examine only the signal, the change in the growth rate that resulted from the change in alkalinity.”

Zhu designed a statistical model that was capable of quantifying the variation that occurred both naturally — in a portion of the reef that was measured as an experimental control — and as a result of the experiment. The data showed that the reef grew about 7 percent faster when seawater acidity approximated that of preindustrial conditions.

The carbon dioxide that is released into the atmosphere from fossil-fuel consumption acts as a greenhouse gas and negatively impacts the world’s oceans, said Carnegie’s Ken Caldeira, the study’s lead scientist. Ocean impacts of carbon dioxide are partially due to overall warming caused by climate change. But in addition, most atmospheric carbon dioxide is eventually absorbed by oceans and reacts with seawater to form an acid that is corrosive to coral reefs, shellfish and other marine life. This process is known as “ocean acidification.”

Caldeira said coral reefs are particularly vulnerable to ocean acidification, because reef architecture is built by the accretion, or buildup, of calcium carbonate through a process called calcification. Calcification becomes increasingly difficult as acid concentrations increase and the surrounding water’s pH decreases. Scientists have predicted that reefs could begin dissolving within the century if acidification continues and reefs switch from carbonate accretion to carbonate dissolution.

Previous studies have demonstrated large-scale declines in coral reefs over recent decades. Work from another team led by Caldeira found that rates of reef calcification were 40 percent lower in 2008 and 2009 than they were during the same season in 1975 and 1976. But it has been difficult to pinpoint exactly how much of the decline is due to acidification and how much is caused by warming, pollution and overfishing.

In the current study, the team manipulated the alkalinity of seawater flowing over a reef flat off Australia’s One Tree Island. They brought the reef’s pH closer to what it would have been in the preindustrial period based on estimates of atmospheric carbon dioxide from the era. They then measured the reef’s calcification in response to this pH increase. They found that calcification rates under these manipulated preindustrial conditions were about 7 percent higher than they are today.

Caldeira said some researchers have proposed increasing the alkalinity of ocean water around coral reefs through geoengineering to save shallow marine ecosystems. The results of the new study show that this idea could be effective, but he said it would likely be impractical to implement on all but the smallest scales.

“The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions,” Caldeira said. “If we don’t take action on this issue very rapidly, coral reefs — and everything that depends on them, including both wildlife and local communities — will not survive into the next century.”

Additional study co-authors include Carnegie’s Lilian Caldeira, Lester Kwiatkowski, Jana Maclaren (also of Stanford University), Yana Nebuchina, Julia Pongratz (now at Max Planck Institute for Meteorology), Katharine Ricke, Kenny Schneider (now at The Hebrew University of Jerusalem) and Marine Sesboue; as well as Jessica Hosfelt and Aaron Ninokawa of the University of California, Davis; Benjamin Mason of Stanford University; Tanya Rivlin of the Hebrew University of Jerusalem; Kathryn Shamberger of both Woods Hole Oceanographic Institution and Texas A&M University; and Kennedy Wolfe of the University of Sydney.

The research was supported by the Carnegie Institution for Science and the Fund for Innovative Climate and Energy Research.

New faculty at Rice

RUType1 1-Color

Greetings from Houston!

I am now a Huxley Faculty Fellow in Department of BioSciences at Rice University.

Biology began at Rice in 1912 with the appointment of Julian Huxley as the biology professor. Julian Huxley was the grandson of Thomas H. Huxley, a biologist himself and champion of Charles Darwin. Julian made many important contributions to the fields of ethology, ecology and cancer research, and was a powerful proponent of neo-Darwinism.

His efforts are commemorated by the Huxley Fellows program, in which recent Ph.D. recipients are appointed for 2-3 year (non-tenure) faculty positions in BioSciences. Huxley Fellows are outstanding early-career scientists who pursue their own independent research programs in ecology and/or evolution while teaching at the graduate and undergraduate levels. Huxley Fellows do not have an advisor (as a postdoc would), but are expected to collaborate broadly across BioSciences. Huxley Fellows are selected as a result of a competitive application process.

Density dependence paper published in Ecology

Zhu K., Woodall C.W., Monteiro J.V.D., Clark J.S. (2015). Prevalence and strength of density-dependent tree recruitment. Ecology. 96, 2319-2327.

My new paper on density dependence, as the cover paper in the cover of September 2015 issue of Ecology.

Figuring the odds of Earth’s global hot streak

GO FIGURE: Figuring the odds of Earth’s global hot streak

Is global change real? How about this year’s temperature record? I helped to calculate the odds–really low, indeed!

The global heat streak of the 21st century can be explained with statistics that defy astronomical odds.

First, the National Oceanic Atmospheric Administration calculates global average temperature going back to 1880. That’s 135 years. So if no other forces were in play and temperatures last year were totally at random, then the odds of 2014 being the warmest on record are 1 in 135. Not too high.

But record and near record heat keep happening. Climate scientists say it’s not random but from heat-trapping gas spewed by the burning of coal, oil and gas. You know, global warming. And one of their many pieces of evidence is how statistically unlikely it is for the world to have warmed so much.

So how likely are these temperatures to be random? The Associated Press consulted with statisticians to calculate the odds of this hot streak happening at random. Here are some statistics and the odds they calculated, with the caveat that high temperatures tend to persist so that can skew odds a bit:

The three hottest years on record—2014, 2010 and 2005—have occurred in the last 10 years. The odds of that happening randomly are 3,341 to 1, calculated John Grego of the University of South Carolina. Kai Zhu of Stanford University, Robert Lund of Clemson University and David Peterson, a retired Duke statistician, agreed.

Nine of the 10 hottest years on record have occurred in the 21st century. The odds of that being random are 650 million to 1, the statisticians said.

Thirteen of the 15 the hottest years on record have occurred in the last 15 years. The odds of that being random are more than 41 trillion to 1, the statisticians said.

All 15 years from 2000 on have been among the top 20 warmest years on record. They said the odds of that are 1.5 quadrillion to 1. A quadrillion is a million billion.

And then there’s the fact that the last 358 months in a row have been warmer than the 20th-century average, according to NOAA. The odds of that being random are so high—a number with more than 100 zeros behind it—that there is no name for that figure, Grego said.

Stanford grant to study Ebola

Zhiyuan Song and I got a seed grant from Center for Innovation in Global Health Ebola Innovation at Stanford. We will use our expertise to solve global health challenges. Abstract below.

Dynamically evaluating and mapping Ebola outbreak risks in West and Central Africa in response to social-environmental changes

Since the first recorded outbreak of human Ebola virus disease in 1976, the Ebola epidemics have evolved from rare small­scale endemics to frequent larger­scale epidemics. The current ongoing outbreak in West Africa is unprecedented in terms of both spread area and infected cases. In addition, for the first time, more than one Ebola epidemics concurred in separated regions independently (West Africa and DRC). This intensified trend of Ebola outbreaks urges a dynamically updated risk evaluation in response to the social­environmental changes in West and Central Africa. In particular, recent studies suggest that deforestation and urbanization could be important drivers for the increases in frequency and size of the outbreaks. To dynamically evaluate the risk, we propose to perform a synthetic analysis to quantify Ebola risks and the effects from land use, wildlife distribution, climate, economic, demographic and public health factors.

Migration paper among most-cited

Zhu K., Woodall C.W., Clark J.S. (2012). Failure to migrate: lack of tree range expansion in response to climate change. Global Change Biology, 18, 1042-1052.

Of all the 608 articles contributing to the journal’s 2013 Impact Factor, this paper is one of the 25 most-cited since publication according to Web of Science®, placing it among the top 4% of articles.

Embarking at Stanford


Really excited to begin postdoc at Carnegie Institution for Science and Stanford University, with Chris Field, Nona Chiariello, and Tad Fukami!

Graduated from Duke

duke_graduationFive years, three degrees!

2009-2014, Ph.D., Environment (Ecology)

2012-2014, M.S., Statistical Science

2013-2014, Certificate in College Teaching

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