May 1, 2019
NASA/Goddard Space Flight
Center
Human-generated greenhouse
gases and atmospheric particles were affecting global drought risk as far back
as the early 20th century, according to a new study.
Human-generated greenhouse
gases and atmospheric particles were affecting global drought risk as far back
as the early 20th century, according to a study from NASA's Goddard Institute
for Space Studies (GISS) in New York City.
The study, published in the
journal Nature, compared predicted and real-world soil moisture data to
look for human influences on global drought patterns in the 20th century.
Climate models predict that a human "fingerprint" -- a global pattern
of regional drying and wetting characteristic of the climate response to
greenhouse gases -- should be visible early in the 1900's and increase over
time as emissions increased. Using observational data such as precipitation and
historical data reconstructed from tree rings, the researchers found that the
real-world data began to align with the fingerprint within the first half of
the 20th century.
The team said the study is the
first to provide historical evidence connecting human-generated emissions and
drought at near-global scales, lending credibility to forward-looking models
that predict such a connection. According to the new research, the fingerprint
is likely to grow stronger over the next few decades, potentially leading to
severe human consequences.
Searching for human
fingerprints
The study's key drought
indicator was the Palmer Drought Severity Index, or PDSI. The PDSI averages
soil moisture over the summer months using data such as precipitation, air
temperature and runoff. While today NASA measures soil moisture from space,
these measurements only date back to 1980. The PDSI provides researchers with
average soil moisture over long periods of time, making it especially useful
for research on climate change in the past.
The team also used drought
atlases: Maps of where and when droughts happened throughout history,
calculated from tree rings. Tree rings' thickness indicates wet and dry years
across their lifespan, providing an ancient record to supplement written and
recorded data.
"These records go back
centuries," said lead author Kate Marvel, an associate research scientist
at GISS and Columbia University. "We have a comprehensive picture of
global drought conditions that stretch back way into history, and they are
amazingly high quality."
Taken together, modern soil
moisture measurements and tree ring-based records of the past create a data set
that the team compared to the models. They also calibrated their data against
climate models run with atmospheric conditions similar to those in 1850, before
the Industrial Revolution brought increases in greenhouse gases and air
pollution.
"We were pretty surprised
that you can see this human fingerprint, this human climate change signal,
emerge in the first half of the 20th century," said Ben Cook, climate
scientist at GISS and Columbia University's Lamont-Doherty Earth Observatory in
New York City. Cook co-led the study with Marvel.
The story changed briefly
between 1950 and 1975, as the atmosphere became cooler and wetter. The team
believes this was due to aerosols, or particles in the atmosphere. Before the
passage of air quality legislation, industry expelled vast quantities of smoke,
soot, sulfur dioxide and other particles that researchers believe blocked
sunlight and counteracted greenhouse gases' warming effects during this period.
Aerosols are harder to model than greenhouse gases, however, so while they are
the most likely culprit, the team cautioned that further research is necessary
to establish a definite link.
After 1975, as pollution
declined, global drought patterns began to trend back toward the fingerprint.
It does not yet match closely enough for the team to say statistically that the
signal has reappeared, but they agree that the data trends in that direction.
Reaching a verdict
What made this study
innovative was seeing the big picture of global drought, Marvel said.
Individual regions can have significant natural variability year to year,
making it difficult to tell whether a drying trend is due to human activity.
Combining many regions into a global drought atlas meant there was a stronger
signal if droughts happened in several places simultaneously.
"If you look at the
fingerprint, you can say, 'Is it getting dry in the areas it should be getting
drier? Is it getting wetter in the areas it should be getting wetter?'"
she said. "It's climate detective work, like an actual fingerprint at a
crime scene is a unique pattern."
Previous assessments from
national and international climate organizations have not directly linked
trends in global-scale drought patterns to human activities, Cook said, mainly
due to lack of data supporting that link. He suggests that, by demonstrating a
human fingerprint on droughts in the past, this study provides evidence that
human activities could continue to influence droughts in the future.
"Part of our motivation
was to ask, with all these advances in our understanding of natural versus
human caused climate changes, climate modeling and paleoclimate, have we
advanced the science to where we can start to detect human impact on
droughts?" Cook said. His answer: "Yes."
Models predict that droughts
will become more frequent and severe as temperatures rise, potentially causing
food and water shortages, human health impacts, destructive wildfires and
conflicts between peoples competing for resources.
"Climate change is not
just a future problem," said Cook. "This shows it's already affecting
global patterns of drought, hydroclimate, trends, variability -- it's happening
now. And we expect these trends to continue, as long as we keep warming the
world."
Story Source:
Journal Reference:
Kate Marvel, Benjamin I. Cook,
Céline J. W. Bonfils, Paul J. Durack, Jason E. Smerdon, A. Park Williams. Twentieth-century
hydroclimate changes consistent with human influence. Nature, 2019; 569
(7754): 59 DOI: 10.1038/s41586-019-1149-8
