From the University of Leeds and the “settled science” department, comes this new idea that combines measurements with a model.
Understanding the climate impact of natural atmospheric particles
An international team of scientists, led by the University of Leeds, has quantified the relationship between natural sources of particles in the atmosphere and climate change.
Their study, published today in Nature Geoscience, shows that the cooling effect of natural atmospheric particles is greater during warmer years and could therefore slightly reduce the amount that temperatures rise as a result of climate change.
Particles in the atmosphere can alter Earth’s climate by absorbing or reflecting sunlight. These particles are often produced by human activities, such as from cars and industry, but there are also naturally occurring particles.
The team combined atmospheric measurements with a computer model to map the effects of two natural particle sources: smoke from forest fires and the gases emitted by trees that can stick together to form tiny particles.
Study lead author Dr Catherine Scott, from the School of Earth and Environment at Leeds, said: “Natural particles can alter the climate, but they are also strongly controlled by it.
“As the Earth warms, plants release more volatile gases from their leaves – these are the gases that, for example, give pine forests a piney smell. Once in the air these gases can form tiny particles. More particles in the atmosphere reflect away the Sun’s energy, which helps to cool the planet.
“This cooling offsets some of the temperature rise and is known as a negative climate feedback. We can think of forests acting as giant air conditioners slightly reducing the warming due to greenhouse gas emissions.”
Study co-author Dominick Spracklen, Professor of Biosphere-Atmosphere Interactions at Leeds, said: “Overall the response of the climate to an initial warming is to amplify that warming, i.e., a positive feedback.
“This natural negative feedback might act to offset a small amount of warming due to climate change but it is not enough to counteract other strong positive feedbacks in the climate system. This means reductions in greenhouse gas emissions are still required to prevent dangerous levels of global warming.
“Our research highlights the need for these complex interactions to be well represented in climate models. The latest generation of models being used for future climate projections include more detail about the way that the atmosphere and the land surface interact than ever before – but it’s important that we can isolate the role that these processes are playing as the climate evolves.”
The research paper, “Substantial large-scale feedbacks between natural aerosols and climate“, is published in Nature Geoscience 4th December 2017. (DOI: 10.1038/s41561-017-0020-5). https://www.nature.com/articles/s41561-017-0020-5
The terrestrial biosphere is an important source of natural aerosol. Natural aerosol sources alter climate, but are also strongly controlled by climate, leading to the potential for natural aerosol–climate feedbacks. Here we use a global aerosol model to make an assessment of terrestrial natural aerosol–climate feedbacks, constrained by observations of aerosol number. We find that warmer-than-average temperatures are associated with higher-than-average number concentrations of large (>100 nm diameter) particles, particularly during the summer. This relationship is well reproduced by the model and is driven by both meteorological variability and variability in natural aerosol from biogenic and landscape fire sources. We find that the calculated extratropical annual mean aerosol radiative effect (both direct and indirect) is negatively related to the observed global temperature anomaly, and is driven by a positive relationship between temperature and the emission of natural aerosol. The extratropical aerosol–climate feedback is estimated to be −0.14 W m−2 K−1 for landscape fire aerosol, greater than the −0.03 W m−2 K−1 estimated for biogenic secondary organic aerosol. These feedbacks are comparable in magnitude to other biogeochemical feedbacks, highlighting the need for natural aerosol feedbacks to be included in climate simulations.