Anthropogenic aerosols exert a cooling influence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only several days, the aerosol forcing responds quickly to emissions. Here, we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow us to robustly conclude that the anthropogenic aerosol effective radiative forcing (ERF) – both aerosol–radiation interactions (ERFari) and aerosol–cloud interactions (ERFaci) – has become less negative globally, i.e. the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the fine-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends in regions with aerosol declines that are consistent with these in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to the year 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m−2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to the Intergovernmental Panel on Climate Change (IPCC).
An increase in effective radiative forcing from human activity is now said to be mostly driven by a decline in aerosol pollution, superseding the effects of CO2 emissions.
The majority of an alleged acceleration in anthropogenic global warming in the 21st century “is driven by changes in the the aerosol [effective radiative forcing] trend, due to aerosol emissions reductions” (Jenkins et al., 2022).
Abstract
Estimates of the anthropogenic effective radiative forcing (ERF) trend have increased by 50% since 2000 (+0.4W/m2/decade in 2000-2009 to +0.6W/m2/decade in 2010-2019), the majority of which is driven by changes in the aerosol ERF trend, due to aerosol emissions reductions. Here we study the extent to which observations of the climate system agree with these ERF assumptions. We use a large ERF ensemble from IPCC’s Sixth Assessment Report (AR6) to attribute the anthropogenic contributions to global mean surface temperature (GMST), top-of-atmosphere radiative flux, and aerosol optical depth observations. The GMST trend has increased from +0.18°C/decade in 2000-2009 to +0.35°C/decade in 2010-2019, coinciding with the anthropogenic warming trend rising from +0.19°C/decade in 2000-2009 to +0.24°C/decade in 2010-2019. This, and observed trends in top-of-atmosphere radiative fluxes and aerosol optical depths support the claim of an aerosol-induced temporary acceleration in the rate of warming. However, all three observation datasets additionally suggest smaller aerosol ERF trend changes are compatible with observations since 2000, since radiative flux and GMST trends are significantly influenced by internal variability over this period. A zero-trend-change aerosol ERF scenario results in a much smaller anthropogenic warming acceleration since 2000, but is poorly represented in AR6’s ERF ensemble. Short-term ERF trends are difficult to verify using observations, so caution is required in predictions or policy judgments that depend on them, such as estimates of current anthropogenic warming trend, and the time remaining to, or the outstanding carbon budget consistent with, 1.5°C warming. Further systematic research focused on quantifying trends and early identification of acceleration or deceleration is required.
The so-called hiatus in global annual average temperature between 2002 – 2014, once controversial to some but now well-established in the peer-reviewed literature, ended in 2014 with the start of a series of record-breaking El Nino events that spiked global temperature with a subsequent fall-back. Now a new study into the effect of man-made aerosol pollution adds to likely reasons for the end of the hiatus, and may point to lower estimates for future global warming.
An international research team writing in the journal Atmospheric Chemistry and Physics, uses satellite data to show that concentrations of aerosol particles have decreased significantly since 2000. This is good news as cleaner air benefits health, but it also reduces particles’ which have a cooling effect on the terrestrial climate.
According to the Intergovernmental Panel on Climate Change (IPCC), by 2019 the global temperature had risen by 1.1 degrees Celsius compared to pre-industrial levels due to increasing greenhouse gasses from burning fossil fuels. At the same time the combustion of fossil fuels emit aerosols which cool our climate by reflecting sunlight and increasing the reflectivity of clouds.
Professor Johannes Quaas, a meteorologist at Leipzig University, and colleagues from Europe, China, and the US have published robust observational evidence of significant reduction of aerosol pollution and improved global air quality.
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When taken together with a couple of super-strong El Nino events which temporarily drove up global temperature (see graph below), the new findings suggest that the global warming hiatus — clearly evident prior to 2014 — may not have ended yet. If NASA’s satellite data are confirmed, it would suggest that much of the very moderate changes in global temperature this century may have been driven primarily by cleaner air and naturally-occurring El Ninos.
Global temperature changes 2000-2022. Source: Met Office/HadCRUT5…
Die kalte Sonne reports on a new aerosol study by Liu et al.
The results are a major blow to the high greenhouse-gas climate sensitivity modelers.
IPCC scientists have a favorite wild card they often use to explain serious model discrepancies: aerosols. Mysterious cooling events in the past are often explained away by aerosols from major volcanic eruptions, for example. They act to filter out sunlight.
ccording to IPCC climate models, the mean global temperature should have risen by 1.5°C since 1850 due to the higher CO2 concentrations. But best estimates show that it has instead risen by only 1.1°C. So what about the missing 0.4°C?
Naturally, the missing 0.4°C of warming since 1850 gets explained by the higher 20th century aerosol levels in the atmosphere – due to the burning of fossil fuels. Air pollution by man over the course of the late 19th century and entire 20th century are said to have dimmed the earth, and thus this explains the 0.4°C less warming.
Surprise: global aerosol emissions have been flat over past 250 years
But now results by a new study appearing in the journal Science Advances by Liu et al tells us that the forcing by aerosols had to have been overestimated by climate modelers. IPCC modelers insisted that 20th century aerosol concentrations were higher than during the pre-industrial times, and this is what kept the climate from warming by 1.5°C.
According to the scientists led by Liu, however, atmospheric aerosols in the preindustrial times were just as high as they were just recently. They were in fact more or less constant over the past 250 years. No change means it could not have been aerosols putting the brakes on temperature rise:
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That’s a real embarrassment for the IPCC modelers. It means CO2 climate sensitivity has been overestimated.
Large amounts of new particles can form in the valleys of the Himalayas from naturally emitted gases and can be transported to high altitudes by the mountain winds and injected into the upper atmosphere.
The emitted particles may eventually affect climate by acting as nuclei for cloud condensation. These new findings about particles formation and sources will contribute to a better understanding of past and future climate.
“To understand how the climate has changed over the last century we need to know as reliably as possible the natural atmospheric conditions before the industrialization,” says Associate Professor Federico Bianchi from the University of Helsinki’s Institute for Atmospheric and Earth System Research (INAR).
In order to do that scientists are looking for pristine locations around the world where human influence is minimal. An international group of researchers has now completed a comprehensive study at the Nepal Climate Observatory at Pyramid station, located in the proximity of the Everest base camp at 5050 m above sea level. There, they were able to investigate the formation of atmospheric particles far from human activities. The results were published today in the prestigious journal Nature Geoscience.
Anthropogenic emissions of SO2 into the troposphere peaked during year 1972 at about 131 Megatonnes. By year 2000, due to worldwide Clean Air Act efforts, SO2 emissions in the West had decreased by approximately 48 Megatonnes. However, during the same time period, emissions elsewhere rose by 23 Megatonnes, for a net worldwide decrease of 25 Megatonnes.
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Figure 1: Global sulfur dioxide emissions by region (North Amer- ica = USA,Canada; East Asia, Japan, China, and South Korea). J.Smith et al., Fig 6.
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It also proves that the IPCC “Graph of Radiative Forcings” is completely incorrect, since it does not include any warming due to the removal of dimming-aerosols from the atmosphere. To be correct, this forcing needs to be included (which will have the effect of completely eliminating any forcing due to CO2). As noted above, all of the warming can be accounted for by the reduction in SO2 emissions.
How much impact does the abundance of cloud condensation nuclei (CCN) aerosols above the oceans have on global temperatures? Rosenfeld et al.analyzed how CCN affect the properties of marine stratocumulus clouds, which reflect much of the solar radiation received by Earth back to space (see the Perspective by Sato and Suzuki). The CCN abundance explained most of the variability in the radiative cooling. Thus, the magnitude of radiative forcing provided by these clouds is much more sensitive to the presence of CCN than current models indicate, which suggests the existence of other compensating warming effects.
Researchers from Leipzig cooperate with scientists from Punta Arenas (Chile) to learn more about the relationship between air pollution, clouds and precipitation.
Leipzig/Punta Arenas. How do airborne particles, so-called aerosols, affect the formation and life cycle of clouds and precipitation? In order to come one step closer to solving this question, atmospheric scientists from the Leibniz Institute for Tropospheric Research (TROPOS) and the Leipzig Institute for Meteorology (LIM) at Leipzig University will observe the atmosphere at one of the cleanest places in the world for at least a year. The choice fell on Punta Arenas because the city is located on a comparable geographical latitude as Germany and will thus enable comparisons between the northern and southern hemispheres. The measurement campaign is part of the International Year of Polar Prediction (YOPP), which aims to improve weather and climate forecasts for the polar regions through intensive measurements.
Danish Professor Henrik Svensmark is a leading physicist of cosmic radiation. At the end of last year he made a presentation at the 12th International Climate Conference in Munich, where he demonstrated that the climate is indeed modulated in large part by cloud cover, which in turn is modulated by solar activity in combination with cosmic rays.
His theory is that cosmic rays, which are extremely fast-flying particles – which originate from dying supernovae – travel through the cosmos, strike the Earth’s atmosphere and have a major impact on cloud cover and thus climate on the Earth’s surface.
This, Svensmark says, has been confirmed in numerous laboratory experiments.
… “Wildfires are such a huge source of aerosol in the atmosphere with a combination of cooling and warming properties, that understanding the delicate balance can have profound consequences on how accurately we can predict future changes,” says Claudio Mazzoleni, professor of physics, and one of the authors of the paper.
As wildfires increase in size and frequency in the world’s arid regions, more aerosol particles could be injected into the free troposphere where they are slower to oxidize, contributing another important consideration to the study of atmospheric science and climate change.
A new paper published by NASA by Tsigaridis and Kanakidou suggests that climate models have missed the forcing effects of organic aerosols, such as VOC’s from trees, oceans, and other sources that combine chemically in the atmosphere to create new compounds. Known as Secondary organic aerosols (SOA), they say “SOA forcing could exceed that of sulfate and black carbon”.
Scientists have quantified the relationship between natural sources of particles in the atmosphere and climate change. Their research 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.Share:
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