Tag Archives: Aerosols

Pollution in the north shrank Lake Chad

FOR IMMEDIATE RELEASE The bad habits of the locals have been blamed for the decline of Lake Chad in Africa but it was pollution from people far away that caused rain patterns to shift. LONDON, 16 June – American scientists have a new explanation for one of the great ecological disasters of the 1980s. The alarming near-disappearance of Lake Chad – a giant body of water that nourished crops in the Sahel region – was, they say, caused by air pollution: old-fashioned smog and soot from factory chimneys and coal-burning power plants in Europe and America. The initial explanation had been a much simpler one, and pinned the guilt on the locals. Lake Chad, which extended over 25,000 square kilometres in the 1960s, shrank to a 20th of its former area by the end of the last century, all because of overgrazing and too great a demand for water for irrigation, geographers had once argued. The consequences for the local peoples of Nigeria, Chad, Cameroon and Niger were devastating, and triggered global concern, especially as the summer rains repeatedly failed and the lake was not seasonally replenished. Another culprit Later, Lake Chad became an awful example of the possible consequences of global warming. In the latest twist in the story, scientists at the University of Washington in the US have pointed to another culprit: the sulphate aerosol. Aerosols pumped from chimneys and exhaust pipes in the developed world scattered in the atmosphere and reflected sunlight back into space, to cool the entire northern hemisphere, the region with the greatest land mass, the highest economic development and the most factory chimneys. In response to a small change in overall conditions the tropical rain belt shifted southwards with a steady decrease in precipitation in the Sahel from the 1950s onward. The lowest ever recorded rainfall in the region was during the early 1980s, “perhaps the most striking precipitation change in the 20th century observational record,” say Yen-Ting Hwang and colleagues in Geophysical Research Letters. In fact, the authors are careful to say this is “in part” an explanation of the drought in the Sahel: complex natural changes have complex causes, and both global climate change and pressure from human population growth remain implicated. Hwang’s study used six decades of continuous data from rain gauges to link the drought to a global shift in tropical rainfall, and then used 26 different climate models to make the link between hemisphere temperatures and the pattern of rainfall. The Sahel was not the only region affected: northern India and parts of South America experienced drier decades, while places at the southern edge of the tropical rain belt, such as north-east Brazil and the African Great Lakes, were wetter than normal. Rain shifts again As clean air legislation passed both in the US and Europe slowly cleared the skies, the northern hemisphere began to warm faster than the southern hemisphere, and the tropical rain belt began to shift north again. A team at the University of California, Berkeley, in April reported in the Journal of Climate, published by the American Meteorological Society, that temperature differences measured over a century coincided with changes in the pattern of tropical rainfall. The largest difference – a drop of about half a degree Celsius in the northern hemisphere in the late 1960s, coincided with a 30-year drought in the Sahel, the growth of the deserts in the Sahara and the failures of the monsoons in India and east Asia. The research is a reminder that climate patterns are sensitive to even very small average shifts in temperature on a very large scale; that what happens in one region can quite dramatically affect conditions in another part of the globe; and that human actions in some of the richest regions of the planet can have cruel consequences for those trying to make a living in the poorest places. Meanwhile, although the rains have returned, Lake Chad is still very much diminished. – Climate News Network

FOR IMMEDIATE RELEASE The bad habits of the locals have been blamed for the decline of Lake Chad in Africa but it was pollution from people far away that caused rain patterns to shift. LONDON, 16 June – American scientists have a new explanation for one of the great ecological disasters of the 1980s. The alarming near-disappearance of Lake Chad – a giant body of water that nourished crops in the Sahel region – was, they say, caused by air pollution: old-fashioned smog and soot from factory chimneys and coal-burning power plants in Europe and America. The initial explanation had been a much simpler one, and pinned the guilt on the locals. Lake Chad, which extended over 25,000 square kilometres in the 1960s, shrank to a 20th of its former area by the end of the last century, all because of overgrazing and too great a demand for water for irrigation, geographers had once argued. The consequences for the local peoples of Nigeria, Chad, Cameroon and Niger were devastating, and triggered global concern, especially as the summer rains repeatedly failed and the lake was not seasonally replenished. Another culprit Later, Lake Chad became an awful example of the possible consequences of global warming. In the latest twist in the story, scientists at the University of Washington in the US have pointed to another culprit: the sulphate aerosol. Aerosols pumped from chimneys and exhaust pipes in the developed world scattered in the atmosphere and reflected sunlight back into space, to cool the entire northern hemisphere, the region with the greatest land mass, the highest economic development and the most factory chimneys. In response to a small change in overall conditions the tropical rain belt shifted southwards with a steady decrease in precipitation in the Sahel from the 1950s onward. The lowest ever recorded rainfall in the region was during the early 1980s, “perhaps the most striking precipitation change in the 20th century observational record,” say Yen-Ting Hwang and colleagues in Geophysical Research Letters. In fact, the authors are careful to say this is “in part” an explanation of the drought in the Sahel: complex natural changes have complex causes, and both global climate change and pressure from human population growth remain implicated. Hwang’s study used six decades of continuous data from rain gauges to link the drought to a global shift in tropical rainfall, and then used 26 different climate models to make the link between hemisphere temperatures and the pattern of rainfall. The Sahel was not the only region affected: northern India and parts of South America experienced drier decades, while places at the southern edge of the tropical rain belt, such as north-east Brazil and the African Great Lakes, were wetter than normal. Rain shifts again As clean air legislation passed both in the US and Europe slowly cleared the skies, the northern hemisphere began to warm faster than the southern hemisphere, and the tropical rain belt began to shift north again. A team at the University of California, Berkeley, in April reported in the Journal of Climate, published by the American Meteorological Society, that temperature differences measured over a century coincided with changes in the pattern of tropical rainfall. The largest difference – a drop of about half a degree Celsius in the northern hemisphere in the late 1960s, coincided with a 30-year drought in the Sahel, the growth of the deserts in the Sahara and the failures of the monsoons in India and east Asia. The research is a reminder that climate patterns are sensitive to even very small average shifts in temperature on a very large scale; that what happens in one region can quite dramatically affect conditions in another part of the globe; and that human actions in some of the richest regions of the planet can have cruel consequences for those trying to make a living in the poorest places. Meanwhile, although the rains have returned, Lake Chad is still very much diminished. – Climate News Network

Warming trees limit warming – a little

FOR IMMEDIATE RELEASE Warmer temperature prompts trees to release aerosols which in turn stimulate cloud formation. And that can help to cool the temperature, at least modestly. LONDON, 1 May – Trees may provide the Earth with a little shade from global warming – indirectly. European and Canadian researchers report that they have found what engineers like to call a negative feedback loop above the forests of Europe and North America. It works like this. Trees – those natural chemical factories that routinely deliver complex aromatic compounds such as rubber, coffee, chocolate, resins, pungent fruits, oils and natural drugs such as quinines – are a permanent source of volatile organic compounds released into the atmosphere. On a hot day, trees release even more conspicuous quantities of terpenes, isoprenes and other compounds into the air. These are wafted higher in the atmosphere and begin to mix, oxidise, or chemically react with other atmospheric gases, aerosols and car and factory exhausts to form increasingly larger particles on which water vapour might condense. This is not a new observation. The Smoky Mountains of Tennessee and North Carolina take their name from the pall of isoprenes discharged from the oaks that cover the hills: the mountains actually look smoky. The aerosols from trees float in the atmosphere and reflect and scatter sunlight and even form cloud droplets. So far, so familiar. But Pauli Paasonen, of the University of Helsinki and the International Institute for Applied Systems Analysis in Laxenburg, Austria, writes in Nature Climate Change that he and 23 colleagues in Finland, Sweden, Germany, Canada and the US decided to assess the overall effect of these aerosols and their contribution to, or impact upon, global warming.

Every little helps

They analysed data from eleven measuring stations spread across the northern hemisphere, from semi-Arctic wilderness to polluted agricultural lands, and worked out how the quantities of cloud condensation nuclei might be linked to air temperatures. They found a clear connection. The warmer the weather, the greater the likelihood that gas emissions from plants would create conditions for the formation of clouds, which in turn would reflect more sunlight back into space, and thus help damp down global warming. That is the good news. The not-so-good news is that these plant gas emissions won’t make a great deal of difference – on a global scale they might counter about 1% of global warming. On a regional scale, however, the effect might be much greater: in heavily forested areas – Finland, Siberia and Canada, for instance – where human emissions of aerosols are anyway relatively slight, plant gas releases might counter as much as 30% of warming. The effect however was not easy to predict, and may not be easy to confirm. The key variable is the boundary layer of the atmosphere at which gases and particles mix and form the nuclei around which cloud droplets might condense, and the height of this boundary changes with weather conditions. “Plants, by reacting to changes in temperature, also moderate these changes”, says Dr Paasonen. “One of the reasons that this phenomenon was not discovered earlier was because these estimates for boundary layer height are very difficult to do.” – Climate News Network

FOR IMMEDIATE RELEASE Warmer temperature prompts trees to release aerosols which in turn stimulate cloud formation. And that can help to cool the temperature, at least modestly. LONDON, 1 May – Trees may provide the Earth with a little shade from global warming – indirectly. European and Canadian researchers report that they have found what engineers like to call a negative feedback loop above the forests of Europe and North America. It works like this. Trees – those natural chemical factories that routinely deliver complex aromatic compounds such as rubber, coffee, chocolate, resins, pungent fruits, oils and natural drugs such as quinines – are a permanent source of volatile organic compounds released into the atmosphere. On a hot day, trees release even more conspicuous quantities of terpenes, isoprenes and other compounds into the air. These are wafted higher in the atmosphere and begin to mix, oxidise, or chemically react with other atmospheric gases, aerosols and car and factory exhausts to form increasingly larger particles on which water vapour might condense. This is not a new observation. The Smoky Mountains of Tennessee and North Carolina take their name from the pall of isoprenes discharged from the oaks that cover the hills: the mountains actually look smoky. The aerosols from trees float in the atmosphere and reflect and scatter sunlight and even form cloud droplets. So far, so familiar. But Pauli Paasonen, of the University of Helsinki and the International Institute for Applied Systems Analysis in Laxenburg, Austria, writes in Nature Climate Change that he and 23 colleagues in Finland, Sweden, Germany, Canada and the US decided to assess the overall effect of these aerosols and their contribution to, or impact upon, global warming.

Every little helps

They analysed data from eleven measuring stations spread across the northern hemisphere, from semi-Arctic wilderness to polluted agricultural lands, and worked out how the quantities of cloud condensation nuclei might be linked to air temperatures. They found a clear connection. The warmer the weather, the greater the likelihood that gas emissions from plants would create conditions for the formation of clouds, which in turn would reflect more sunlight back into space, and thus help damp down global warming. That is the good news. The not-so-good news is that these plant gas emissions won’t make a great deal of difference – on a global scale they might counter about 1% of global warming. On a regional scale, however, the effect might be much greater: in heavily forested areas – Finland, Siberia and Canada, for instance – where human emissions of aerosols are anyway relatively slight, plant gas releases might counter as much as 30% of warming. The effect however was not easy to predict, and may not be easy to confirm. The key variable is the boundary layer of the atmosphere at which gases and particles mix and form the nuclei around which cloud droplets might condense, and the height of this boundary changes with weather conditions. “Plants, by reacting to changes in temperature, also moderate these changes”, says Dr Paasonen. “One of the reasons that this phenomenon was not discovered earlier was because these estimates for boundary layer height are very difficult to do.” – Climate News Network

Volcanoes may mask 'lost' warming

EMBARGOED until 0001 GMT on Monday 4 March
Scientists believe that emissions of sulphur dioxide from erupting volcanoes may be masking the effect of greenhouse gases and so keeping temperature rises artificially low.

LONDON, 4 March – Climate scientists think they may have found at least part of the answer to a conundrum which has been puzzling them recently – why the atmosphere has not warmed as much as expected over the last decade or so.

A team led by the University of Colorado-Boulder (CU-Boulder) thinks the reason may be emissions of sulphur dioxide (SO2), a known inhibitor of atmospheric warming, from many of the world’s volcanoes.

The puzzle is why the global average temperature has not increased as expected in step with rising greenhouse gas emissions. This has led some to suggest that global warming itself is faltering, and with it the entire scientific justification for action to stabilise the climate.

The chair of the Intergovernmental Panel on Climate Change, Dr R K Pachauri, was reported in The Australian on 22 February as having acknowledged “a 17-year pause in global temperature rises, confirmed recently by Britain’s Met Office”, but “said it would need to last ’30 to 40 years at least’ to break the long-term global warming trend”.

Most SO2 emissions are from fossil fuel burning at power plants and in industry, with smaller amounts coming from railways, large vessels and some industrial processes.

Volcanoes downplayed

 

Small quantities of the emissions eventually rise into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulphuric acid and water particles that reflect sunlight back into space, cooling the planet.

Scientists have known for years that this cooling mechanism from a range of aerosols is helping to prevent global average temperatures rising as much as they otherwise would under the influence of greenhouse gases, but it appears they have underestimated the effect of volcanic SO2.

India and China are estimated to have increased their industrial SO2 emissions by about 60% between 2000 and 2010 through coal burning.

But the study, published online in Geophysical Research Letters, suggests it is volcanic eruptions, not Asia’s emissions, that are largely responsible for the warming slowdown.

The study’s lead author, Ryan Neely, said previous observations had suggested that increases in stratospheric aerosols as a whole since 2000 had cancelled out as much as 25% of the warming from greenhouse gases.

“This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet”, said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration (NOAA).

Negligible overall effect

 

The new study relies on long-term measurements of changes in the stratospheric aerosol layer’s “optical depth,” which is a measure of transparency, said Neely. Since 2000 the optical depth in the layer has increased by about 4 to 7%, meaning it is slightly more opaque now than it was.

“The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth’s climate”,  said Professor Brian Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences.

“But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.”

“This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.”

While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, said Toon. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tonnes of SO2 that cooled the Earth slightly for the next few years. – Climate News Network

EMBARGOED until 0001 GMT on Monday 4 March
Scientists believe that emissions of sulphur dioxide from erupting volcanoes may be masking the effect of greenhouse gases and so keeping temperature rises artificially low.

LONDON, 4 March – Climate scientists think they may have found at least part of the answer to a conundrum which has been puzzling them recently – why the atmosphere has not warmed as much as expected over the last decade or so.

A team led by the University of Colorado-Boulder (CU-Boulder) thinks the reason may be emissions of sulphur dioxide (SO2), a known inhibitor of atmospheric warming, from many of the world’s volcanoes.

The puzzle is why the global average temperature has not increased as expected in step with rising greenhouse gas emissions. This has led some to suggest that global warming itself is faltering, and with it the entire scientific justification for action to stabilise the climate.

The chair of the Intergovernmental Panel on Climate Change, Dr R K Pachauri, was reported in The Australian on 22 February as having acknowledged “a 17-year pause in global temperature rises, confirmed recently by Britain’s Met Office”, but “said it would need to last ’30 to 40 years at least’ to break the long-term global warming trend”.

Most SO2 emissions are from fossil fuel burning at power plants and in industry, with smaller amounts coming from railways, large vessels and some industrial processes.

Volcanoes downplayed

 

Small quantities of the emissions eventually rise into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulphuric acid and water particles that reflect sunlight back into space, cooling the planet.

Scientists have known for years that this cooling mechanism from a range of aerosols is helping to prevent global average temperatures rising as much as they otherwise would under the influence of greenhouse gases, but it appears they have underestimated the effect of volcanic SO2.

India and China are estimated to have increased their industrial SO2 emissions by about 60% between 2000 and 2010 through coal burning.

But the study, published online in Geophysical Research Letters, suggests it is volcanic eruptions, not Asia’s emissions, that are largely responsible for the warming slowdown.

The study’s lead author, Ryan Neely, said previous observations had suggested that increases in stratospheric aerosols as a whole since 2000 had cancelled out as much as 25% of the warming from greenhouse gases.

“This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet”, said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration (NOAA).

Negligible overall effect

 

The new study relies on long-term measurements of changes in the stratospheric aerosol layer’s “optical depth,” which is a measure of transparency, said Neely. Since 2000 the optical depth in the layer has increased by about 4 to 7%, meaning it is slightly more opaque now than it was.

“The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth’s climate”,  said Professor Brian Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences.

“But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.”

“This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.”

While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, said Toon. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tonnes of SO2 that cooled the Earth slightly for the next few years. – Climate News Network

Volcanoes may mask ‘lost’ warming

EMBARGOED until 0001 GMT on Monday 4 March Scientists believe that emissions of sulphur dioxide from erupting volcanoes may be masking the effect of greenhouse gases and so keeping temperature rises artificially low. LONDON, 4 March – Climate scientists think they may have found at least part of the answer to a conundrum which has been puzzling them recently – why the atmosphere has not warmed as much as expected over the last decade or so. A team led by the University of Colorado-Boulder (CU-Boulder) thinks the reason may be emissions of sulphur dioxide (SO2), a known inhibitor of atmospheric warming, from many of the world’s volcanoes. The puzzle is why the global average temperature has not increased as expected in step with rising greenhouse gas emissions. This has led some to suggest that global warming itself is faltering, and with it the entire scientific justification for action to stabilise the climate. The chair of the Intergovernmental Panel on Climate Change, Dr R K Pachauri, was reported in The Australian on 22 February as having acknowledged “a 17-year pause in global temperature rises, confirmed recently by Britain’s Met Office”, but “said it would need to last ’30 to 40 years at least’ to break the long-term global warming trend”. Most SO2 emissions are from fossil fuel burning at power plants and in industry, with smaller amounts coming from railways, large vessels and some industrial processes.

Volcanoes downplayed

  Small quantities of the emissions eventually rise into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulphuric acid and water particles that reflect sunlight back into space, cooling the planet. Scientists have known for years that this cooling mechanism from a range of aerosols is helping to prevent global average temperatures rising as much as they otherwise would under the influence of greenhouse gases, but it appears they have underestimated the effect of volcanic SO2. India and China are estimated to have increased their industrial SO2 emissions by about 60% between 2000 and 2010 through coal burning. But the study, published online in Geophysical Research Letters, suggests it is volcanic eruptions, not Asia’s emissions, that are largely responsible for the warming slowdown. The study’s lead author, Ryan Neely, said previous observations had suggested that increases in stratospheric aerosols as a whole since 2000 had cancelled out as much as 25% of the warming from greenhouse gases. “This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet”, said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration (NOAA).

Negligible overall effect

  The new study relies on long-term measurements of changes in the stratospheric aerosol layer’s “optical depth,” which is a measure of transparency, said Neely. Since 2000 the optical depth in the layer has increased by about 4 to 7%, meaning it is slightly more opaque now than it was. “The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth’s climate”,  said Professor Brian Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences. “But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.” “This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.” While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, said Toon. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tonnes of SO2 that cooled the Earth slightly for the next few years. – Climate News Network

EMBARGOED until 0001 GMT on Monday 4 March Scientists believe that emissions of sulphur dioxide from erupting volcanoes may be masking the effect of greenhouse gases and so keeping temperature rises artificially low. LONDON, 4 March – Climate scientists think they may have found at least part of the answer to a conundrum which has been puzzling them recently – why the atmosphere has not warmed as much as expected over the last decade or so. A team led by the University of Colorado-Boulder (CU-Boulder) thinks the reason may be emissions of sulphur dioxide (SO2), a known inhibitor of atmospheric warming, from many of the world’s volcanoes. The puzzle is why the global average temperature has not increased as expected in step with rising greenhouse gas emissions. This has led some to suggest that global warming itself is faltering, and with it the entire scientific justification for action to stabilise the climate. The chair of the Intergovernmental Panel on Climate Change, Dr R K Pachauri, was reported in The Australian on 22 February as having acknowledged “a 17-year pause in global temperature rises, confirmed recently by Britain’s Met Office”, but “said it would need to last ’30 to 40 years at least’ to break the long-term global warming trend”. Most SO2 emissions are from fossil fuel burning at power plants and in industry, with smaller amounts coming from railways, large vessels and some industrial processes.

Volcanoes downplayed

  Small quantities of the emissions eventually rise into the stratospheric aerosol layer of the atmosphere, where chemical reactions create sulphuric acid and water particles that reflect sunlight back into space, cooling the planet. Scientists have known for years that this cooling mechanism from a range of aerosols is helping to prevent global average temperatures rising as much as they otherwise would under the influence of greenhouse gases, but it appears they have underestimated the effect of volcanic SO2. India and China are estimated to have increased their industrial SO2 emissions by about 60% between 2000 and 2010 through coal burning. But the study, published online in Geophysical Research Letters, suggests it is volcanic eruptions, not Asia’s emissions, that are largely responsible for the warming slowdown. The study’s lead author, Ryan Neely, said previous observations had suggested that increases in stratospheric aerosols as a whole since 2000 had cancelled out as much as 25% of the warming from greenhouse gases. “This new study indicates it is emissions from small to moderate volcanoes that have been slowing the warming of the planet”, said Neely, a researcher at the Cooperative Institute for Research in Environmental Sciences, a joint venture of CU-Boulder and the National Oceanic and Atmospheric Administration (NOAA).

Negligible overall effect

  The new study relies on long-term measurements of changes in the stratospheric aerosol layer’s “optical depth,” which is a measure of transparency, said Neely. Since 2000 the optical depth in the layer has increased by about 4 to 7%, meaning it is slightly more opaque now than it was. “The biggest implication here is that scientists need to pay more attention to small and moderate volcanic eruptions when trying to understand changes in Earth’s climate”,  said Professor Brian Toon of CU-Boulder’s Department of Atmospheric and Oceanic Sciences. “But overall these eruptions are not going to counter the greenhouse effect. Emissions of volcanic gases go up and down, helping to cool or heat the planet, while greenhouse gas emissions from human activity just continue to go up.” “This paper addresses a question of immediate relevance to our understanding of the human impact on climate,” said Neely. “It should interest those examining the sources of decadal climate variability, the global impact of local pollution and the role of volcanoes.” While small and moderate volcanoes mask some of the human-caused warming of the planet, larger volcanoes can have a much bigger effect, said Toon. When Mount Pinatubo in the Philippines erupted in 1991, it emitted millions of tonnes of SO2 that cooled the Earth slightly for the next few years. – Climate News Network