Earth is on a budget – an energy budget. Our planet is constantly trying to balance the flow of energy in and out of Earth’s system. But human activities are throwing that off balance, causing our planet to warm in response.
Radiative energy enters Earth’s system from the sunlight that shines on our
planet. Some of this energy reflects off of Earth’s surface or atmosphere
back into space. The rest gets absorbed, heats the planet, and is then emitted
as thermal radiative energy the same way that black asphalt gets hot and
radiates heat on a sunny day. Eventually this energy also heads toward space,
but some of it gets re-absorbed by clouds and greenhouse gases in the atmosphere.
The absorbed energy may also be emitted back toward Earth, where it will warm
the surface even more.
Adding more components that absorb radiation – like greenhouse gases – or
removing those that reflect it – like aerosols – throws off Earth’s
energy balance, and causes more energy to be absorbed by Earth instead of
escaping into space. This is called a radiative forcing, and it’s the dominant
way human activities are affecting the climate.
A simplified animation
of Earth's planetary energy balance: A planet’s energy budget is balanced
between incoming (yellow) and outgoing radiation (red). On Earth, natural and
human-caused processes affect the amount of energy received as well as emitted
back to space. This study filters out variations in Earth’s energy budget due
to feedback processes, revealing the energy changes caused by aerosols and
greenhouse gas emissions.
Credits: NASA's Goddard
Space Flight Center Conceptual Image Lab
Download the panels in this GIF from NASA Goddard's
Scientific Visualization Studio
Climate modelling predicts that human activities are causing the release of
greenhouse gases and aerosols that are affecting Earth’s energy budget. Now, a
NASA study has confirmed these predictions with direct observations for the
first time: radiative forcings are increasing due to human actions, affecting
the planet’s energy balance and ultimately causing climate change. The paper was published
online March 25, 2021, in the journal Geophysical
Research Letters.
“This is the first calculation of the total radiative forcing of Earth
using global observations, accounting for the effects of aerosols and
greenhouse gases,” said Ryan Kramer, first author on the paper and a researcher
at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the
University of Maryland, Baltimore County. “It’s direct evidence that human
activities are causing changes to Earth’s energy budget.”
NASA’s Clouds and the Earth’s
Radiant Energy System (CERES) project studies
the flow of radiation at the top of Earth’s atmosphere. A series of CERES
instruments have continuously flown on satellites since 1997. Each measures how
much energy enters Earth’s system and how much leaves, giving the overall net
change in radiation. That data, in combination with other data sources such as
ocean heat measurements, shows that there’s an energy imbalance on our planet.
“But it doesn’t tell us what factors are causing changes in the energy
balance,” said Kramer.
NASA’s CERES instruments
monitor the energy balance of Earth. This image shows how the difference
between incoming and reflected shortwave energy - absorbed solar energy - can
be compared to emitted longwave radiation to determine Earth’s net change in
energy.
Credits: NASA’s
Scientific Visualization Studio
Download this image from NASA Goddard's Scientific
Visualization Studio
This study used a new technique to parse out how much of the total energy
change is caused by humans. The researchers calculated how much of the
imbalance was caused by fluctuations in factors that are often naturally
occurring, such as water vapor, clouds, temperature and surface albedo (essentially
the brightness or reflectivity of Earth’s surface). For example, the Atmospheric Infrared Sounder (AIRS) instrument on
NASA’s Aqua satellite measures water vapor in Earth’s atmosphere. Water vapor
absorbs energy in the form of heat, so changes in water vapor will affect how
much energy ultimately leaves Earth’s system. The researchers calculated the
energy change caused by each of these natural factors, then subtracted the
values from the total. The portion leftover is the radiative forcing.
The team found that human activities have caused the radiative forcing on
Earth to increase by about 0.5 Watts per square meter from 2003 to 2018. The
increase is mostly from greenhouse gases emissions from things like power
generation, transport and industrial manufacturing. Reduced reflective aerosols
are also contributing to the imbalance.
The new technique is computationally faster than previous model-based
methods, allowing researchers to monitor radiative forcing in almost real time.
The method could be used to track how human emissions are affecting the
climate, monitor how well various mitigation efforts are working, and evaluate
models to predict future changes to the climate.
“Creating a direct record of radiative forcing calculated from observations
will allow us to evaluate how well climate models can simulate these forcings,”
said Gavin Schmidt, director of NASA’s Goddard Institute of Space Studies (GISS) in New York City.
“This will allow us to make more confident projections about how the climate
will change in the future.”
Header image caption: A NASA supercomputer model shows how
greenhouse gases like carbon dioxide (CO2) – a key driver of global
warming – fluctuate in Earth’s atmosphere throughout the year. Higher
concentrations are shown in red. Credits: NASA’s Scientific
Visualization Studio / NASA’s Global Modeling and Assimilation Office. Download
this image from NASA Goddard's Scientific Visualization Studio.
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