The impact of atmospheric aerosol on climate

Anatoli Bogdan

Institute of Physical Chemistry, University of Innsbruck Austria

andDepartment of Physics, University of Helsinki

Finland

Contents

Introduction - what is climate?

Factors which impact on climate - atmospheric aerosol - air pollution - albedo - What is permafrost? - Was Global Warming in the past?

The climate where we live is called regional climate.

The regional climate is the average weather in a place over several decades. To describe the regional climate people tell what the temperatures are like over the seasons, how windy it is, and how much rain or snow falls etc.

What is climate?

The regional climate depends on many factors including the amount of sunlight it receives, its height above sea level, the shape of the land, and how close it is to oceans.

Since the equator receives more sunlight than the poles, climate varies depending on distance from the equator.

However, we can also think about the climate of an entire planet. Global climate is a description of the climate of a planet as a whole, with all the regional differences averaged.

Overall, global climate depends on the amount of energy received by the Sun and the amount of energy that is trapped in the system. Scientists, who study Earth's climate and climate change, study the factors affecting the climate.

While the weather can change in a few hours, climate changes over longer periods of time. Climate events, like El Nino, happen over several years, small-scale fluctuations happen over decades, and larger climate changes happen over hundreds and thousands of years.

Currently climate is changing. Our Earth is warming more quickly than it has in the past. Hot summer days may be quite typical of climates in many regions, but global warming is causing Earth's average global temperature to increase. The amount of solar radiation, the chemistry of the atmosphere, clouds, and the biosphere all affect Earth's climate.

As the Sun warms the equator more than the poles, climate varies with latitude. This image shows how sea surface temperatures change at different latitudes. Red colors indicate warmer ocean water, blues and purples indicate cooler ocean water. NOAA (http://windows2universe.org/earth/climate/cli_define.html)

http://earthobservatory.nasa.gov/Features/Aerosols/

Some aerosol particles are a natural part of the atmosphere. Such aerosol particles are coming from erupting volcanoes, sea salt, and wildfires.

However, burning of fossil fuels like coal, oil, and gas produce much more aerosol particles loose in the air. Aerosol particles are a part of air pollution. They are dangerous to human health and they also dampen the effect of global warming.

Aerosols in the atmosphere can change the amount of solar energy that is reflected away from Earth.

Different types of aerosols react differently when hit with sunlight.

Sea salt aerosol particles reflect sunlight back out into space.

Black carbon particles from burning of wood or fossil fuels absorb most of the sunlight that hits them.

Aerosol particles help clouds formation and clouds have an impact on climate.

The millions of cloud droplets need a little particle, like an aerosol, to condense upon.

More aerosols can create more clouds.

Different types of clouds may have different impacts on climate and this is a topic that scientists are still exploring.

But in general, clouds reflect incoming solar radiation back out to space.

http://earthobservatory.nasa.gov/Features/Aerosols/

Scientists think that over most of the last century the overall effect of the added aerosols was a reduced amount of global warming.

Today, however, as new technologies have allowed factories, power plants, and automobiles to release less air pollution into the atmosphere, the amount of aerosols has dropped.

The reduction of air pollution is a good thing because the air pollution is a problem for human health.

However, this has also means that the pace of global warming is likely to increase.

Air Pollution

Air pollution over Mexico City Photo courtesy of Nancy A. Marley

http://windows2universe.org/earth/Atmosphere/airpollution_intro.html

What do smog, acid rain, carbon monoxide, fossil fuel exhausts, and tropospheric ozone have in common?

They are all examples of air pollution. Air pollution is not new.

As far back as the 13 th century, people started complaining about coal dust and soot in the air over London.

Since the beginning of the industrial revolution in the late 1700s, we have been changing the Earth’s atmosphere and its chemistry. As industry spread across the globe, so did air pollution.

Satellite image of particulate pollution over Beijing, China. Click on image for full sizeNASA image courtesy the MODIS Rapid Response Team

http://windows2universe.org/earth/climate/cli_aerosols.html

Air pollution is made up of solid particles and chemicals.

Natural processes impacting the atmosphere include volcanoes, biological decay, and dust storms.

Plants, trees, and grass release volatile organic compounds (VOCs), such as methane, into the air.

We are more concerned with human-made pollution since we have the ability to control it. The pollutants include carbon monoxide, sulfur dioxide, VOCs, and nitrogen oxides. The largest source of human-made pollution is the burning of fossil fuels, including coal, oil, and gas, in our homes, factories, and cars.

Air pollution is either primary or secondary.

Primary pollution is put directly to the air, such as smoke and car exhausts.

Secondary pollution forms in the air when chemical reactions changes primary pollutants. The formation of tropospheric ozone is an example of secondary air pollution.

The atmosphere is a complex, dynamic and fragile system. Concern is growing about the global effects of air pollution, especially climate change. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health.

Albedo

What is albedo?

The fraction of energy reflected by a surface is called albedo. Albedo is measured on a scale from zero to one (or sometimes as a percent).

Very dark colors have an albedo close to zero (or close to 0%). Very light colors have an albedo close to one (or close to 100%).

http://windows2universe.org/earth/albedo.html

Because much of the land surface and oceans are dark in color, they have a low albedo. They absorb a large amount of the solar energy that gets to them, reflecting only a small fraction of it.

Forests have low albedo, near 0.15.

Snow and ice, on the other hand, are very light in color. They have very high albedo, as high as 0.8 or 0.9, and reflect most of the solar energy that gets to them, absorbing very little.

The albedo of all these different surfaces combined is called the planetary albedo.

Earth’s planetary albedo is about 0.31. That means that about a third of the solar energy that gets to Earth is reflected out to space and about two thirds is absorbed.

The Moon’s albedo is 0.07, meaning that only 7% of the solar energy that gets to it is reflected.

The albedo of distant planets, so distant that they are difficult to study with telescopes, can be a very helpful to scientists trying to figure out what a planet is made of.

Earth’s climate depends on the amount of solar radiation that is reflected back out to space and the amount that is absorbed.

If Earth’s climate is colder and there is more snow and ice on the planet, then more solar radiation is reflected back out to space and the climate gets even cooler.

On the other hand, when warming causes snow and ice to melt, darker colored Earth surface and ocean are exposed and less solar energy is reflected out to space causing even more warming. This is known as the ice-albedo feedback.

Clouds have an important effect on albedo too. They have a high albedo and reflect a large amount of solar energy out to space.

Different types of clouds reflect different amounts of solar energy.

If there were no clouds, Earth’s average albedo would drop by half.

Effects of Climate Change Today

Coral reefs, like this one northwest of Bali, Indonesia, are very sensitive to changes in seawater temperature.

http://windows2universe.org/earth/climate/cli_effects.html

Sea level is rising.

During the 20th century, sea level rose about 15 cm due to melting glacier ice and expansion of warmer seawater.

Models predict that sea level may rise as much as 59 cm during the 21st Century, threatening coastal communities, wetlands, and coral reefs.

Arctic sea ice is melting. The summer thickness of sea ice is about half of what it was in 1950. Melting ice may lead to changes in ocean circulation. Plus melting sea ice is speeding up warming in the Arctic.

Glaciers and permafrost are melting. Over the past 100 years, mountain glaciers in all areas of the world have decreased in size and so has the amount of permafrost in the Arctic. Greenland's ice sheet is melting faster too.

Sea-surface temperatures are warming. Warmer waters in the shallow oceans have contributed to the death of about a quarter of the world's coral reefs in the last few decades. Many of the coral animals died after weakened by bleaching, a process tied to warmed waters. Heavier rainfall cause flooding in many regions. Warmer temperatures have led to more intense rainfall events in some areas. This can cause flooding.

Extreme drought is increasing. Higher temperatures cause a higher rate of evaporation and more drought in some areas of the world.

Ecosystems are changing. As temperatures warm, species may either move to a cooler habitat or die. Species that are particularly vulnerable include endangered species, coral reefs, and polar animals. Warming has also caused changes in the timing of spring events and the length of the growing season.

Hurricanes have changed in frequency and strength. There is evidence that the number of intense hurricanes has increased in the Atlantic since 1970. Scientists continue to study whether climate is the cause.

More frequent heat waves. It is likely that heat waves have become more common in more areas of the world.

Warmer temperatures affect human health. There have been more deaths due to heat waves and more allergy attacks as the pollen season grows longer. There have also been some changes in the ranges of animals that carry disease like mosquitoes.

Seawater is becoming more acidic. Carbon dioxide dissolving into the oceans, is making seawater more acidic. There could be impacts on coral reefs and other marine life.

http://windows2universe.org/earth/Atmosphere/images/ca_heat_wave_lg_jpg_image.html

Heat Waves

In the middle of the summer, the weather is very hot – hotter than normal. Such hot period lasts for days. It is hot at night too.

This image, taken by the MODIS sensor on the Aqua satellite, shows the temperatures of California and Nevada during a spring heat wave in May 2004.

Heat waves are a danger to human health – causing heat stroke, heat exhaustion, cramps, and other ailments. They also cause crops to fail and can help start wildfires in dry areas.

How do they form? Sometimes, the jet stream, a flow of air through the mid-latitudes, can bring unusually warm air into a region. If the warm air stays for a while, it can cause a heat wave. The heat is able to persist if there aren’t rain and clouds to cool things off.

The heat-trapping ability of cities, known as the urban heat island effect, can make a heat wave warmer and longer.

Warming of the Polar Regions

These images from NASA’s ICESat satellite show the difference in ice cover in the Arctic between 1980 (top) and 2003 (bottom).

http://windows2universe.org/earth/polar/polar_climate.html

Was Global Warming in the past?

Ancient Global Warming

Earth’s climate is warming quickly now. We know that this has to do with additional greenhouse gases (CO2, H2O, CH4 etc.) in the atmosphere and other global changes. But there is a lot we don’t yet know about how warming will affect our planet. How could we know? We’ve never been through this before, have we?

Actually, even through we humans have never experienced fast global warming, our planet has. And our planet keeps records of what happened. The oldest records that the Earth keeps are in its rocks.

Looking through those records of our planet, geologists search for evidence of ancient climate changes in very old sedimentary rocks. The geologists are interested in learning more how and why rapid global warming happened 635 million years ago. Scientists, collected hundreds of sediment samples from rocks in South Australia. Each sample of sediment was studied with stable isotope analysis, an important tool used to understand climates of the past.

Earth had been covered by a thick ice sheet for millions of years before the warming started 635 million years ago. The research suggests that a little warming caused the ice sheets to collapse. This released a large amount of the greenhouse gas methane into the atmosphere, which had been in a frozen icy form under the ice sheets. The methane increased global warming rapidly.

Today, methane is in Arctic permafrost and beneath the oceans. Researchers believe that these sinks of methane will remain where they are unless triggered by global warming. It's possible that very little warming could unleash this trapped methane, which could warm the Earth tens of degrees.

Permafrost is ground that is below the freezing point of water (0°C) for two or more years. Permafrost is found at high latitudes like the Arctic and Antarctic. It is also common at high altitudes - like mountainous areas – wherever the climate is cold.

Today about 20% of the land surface is frozen permafrost.

Permafrost is considered part of the cryosphere because it is frozen.

However, it is also often considered part of the geosphere because it contains rocks and soils.