Pristine environments offer a window to our cloudy past

Date:
July 27, 2020
Source:
University of Leeds
Summary:
A new study uses satellite data over the Southern Hemisphere
to understand global cloud composition during the industrial
revolution.

This research tackles one of the largest uncertainties in today's
climate models -- the long-term effect of tiny atmospheric particles
on climate change.



FULL STORY ==========================================================================
A new study uses satellite data over the Southern Hemisphere to understand global cloud composition during the industrial revolution. This research tackles one of the largest uncertainties in today's climate models --
the long- term effect of tiny atmospheric particles on climate change.


========================================================================== Climate models currently include the global warming effect of greenhouse
gases as well as the cooling effects of atmospheric aerosols. The tiny particles that make up these aerosols are produced by human-made sources
such as emissions from cars and industry, as well as natural sources
such as phytoplankton and sea spray.

They can directly influence the flow of sunlight and heat within the
Earth's atmosphere as well as interact with clouds. One of the ways
that they do this is by bolstering clouds' ability to reflect sunlight
back into space by increasing their droplet concentration. This in turn
cools the planet. The amount of sunlight that is reflected to space is
referred to Earth's albedo.

However, there has been extremely limited understanding of how aerosol concentration has changed between early-industrial times and the
present day.

This lack of information restricts the ability of climate models
to accurately estimate the long-term effects of aerosols on global
temperatures -and how much of an effect they could have in the future.

Now, an international study led by the Universities of Leeds and
Washington has recognised that remote, pristine parts of the Southern Hemisphere provide a window into what the early-industrial atmosphere
looked like.

The team used satellite measurements of cloud droplet concentration in the atmosphere over the Northern Hemisphere -- heavily polluted with today's industrial aerosols -- and over the relatively pristine Southern Ocean.



==========================================================================
They used these measurements to quantify the possible changes due to
industrial aerosols in Earth's albedo since 1850.

The results, published today in the journal PNAS, suggest that
early-industrial aerosol concentrations and cloud droplet numbers
were much higher than is currently estimated by many global climate
models. This could mean that human- generated atmospheric aerosols are
not having as strong a cooling effect as some climate models estimate. The study suggests that the effect is likely to be more moderate.

Co-lead author, Daniel McCoy, Research Fellow in the School of Earth
and Environment at Leeds, said: "Limitations in our ability to measure
aerosols in the early-industrial atmosphere have made it hard to reduce uncertainties in how much warming there will be in the 21st century.

"Ice cores provide carbon dioxide concentrations from millennia in the
past, but aerosols don't hang around in the same way. One way that we
can try to look back in time is to examine a part of the atmosphere that
we haven't polluted yet.

"These remote areas allow us a glimpse into our past and this helps us understand the climate record and improve our predictions of what will
happen in the future." Co-lead author, Isabel McCoy, from the Atmospheric Sciences Department at Washington, said: "One of the biggest surprises
for us was how high the concentration of cloud droplets is in Southern
Ocean clouds. The way that the cloud droplet concentration increases in summertime tells us that ocean biology is playing an important role in
setting cloud brightness in unpolluted oceans now and in the past.



==========================================================================
"We see high cloud droplet concentrations in satellite and aircraft observations, but not in climate models. This suggests that there are
gaps in the model representation of aerosol-cloud interactions and
aerosol production mechanisms in pristine environments.

"As we continue to observe pristine environments through satellite,
aircraft, and ground platforms, we can improve the representation of the complex mechanisms controlling cloud brightness in climate models and
increase the accuracy of our climate projections." Co-author Leighton
Regayre, a Research Fellow also from the School of Earth and Environment
at Leeds, said: "The science supporting our climate models is improving
all the time. These models are tackling some of the most pressing and
complex environmental questions of the modern era and climate scientists
have always been up front about the fact that uncertainties exist.

"We are only going to reach the answers we need to combat global warming
by regularly interrogating the science. Our team used millions of variants
of a model to explore all the potential uncertainties, the equivalent
of having a clinical trial with millions of participants.

"We hope our findings, along with studies on the detailed process of
aerosol production and aerosol-cloud interactions in pristine environments
that our work has motivated, will help guide the development of the next generation of climate models."

========================================================================== Story Source: Materials provided by University_of_Leeds. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Isabel L. Mccoy, Daniel T. Mccoy, Robert Wood, Leighton Regayre,
Duncan
Watson-Parris, Daniel P. Grosvenor, Jane P. Mulcahy, Yongxiang Hu,
Frida A.-M. Bender, Paul R. Field, Kenneth S. Carslaw, and Hamish
Gordon. The hemispheric contrast in cloud microphysical properties
constrains aerosol forcing. PNAS, 2020 DOI: 10.1073/pnas.1922502117 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200727154216.htm

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