Global dimming

Sometimes the sun looks less bright.

This is called global dimming. It happens because of dirty air. Tiny bits of dust and smoke float up. They act like little mirrors. They bounce the sun away.

This can help keep our world cool. But it can also change our weather. It can make it harder for rain to fall. Since the 1990s, the air has gotten cleaner. Now, the sun looks brighter again.

This is called global brightening.

Sometimes the sun looks less bright. This is called global dimming.

It happens when tiny bits of pollution float in the air. These bits are called aerosols.

One main type is sulfate aerosols. These come from burning fuels like coal. They act like tiny mirrors. They reflect sunlight back into space. This can help keep the Earth cool. It can even hide some global warming.

But dimming can also change our weather. It can lower evaporation. This might lead to less rain in some places.

Another type of pollution is black carbon. People often call this soot. It comes from forest fires and engines.

Since the 1990s, many countries have cleaned their air. This has led to global brightening. This means more sunlight reaches us now. This change can make global warming happen faster. Scientists use models to study these changes. They want to see how our weather will act in the future.

Global dimming is a name for when less sunlight reaches the Earth's surface.

This happens because of tiny particles floating in the air called aerosols. These particles are parts of air pollution. Most of them are sulfate aerosols. These act like tiny mirrors that reflect light away. This process can actually have a cooling effect on our planet. It has even hidden some of the warming caused by climate change.

There are two main ways these particles work. First, they reflect sunlight directly back into space. Second, they change how clouds work. These particles act as nuclei, which are tiny centers for water to gather around. When there are more particles, clouds form with more but smaller water droplets. These clouds become much more reflective. This means even more sunlight bounces off the clouds instead of hitting the ground.

Scientists began to see these changes in the 1950s. They started taking systematic measurements of sunlight during that time. In the 1980s, a researcher named Atsumu Ohmura found something important. He saw that sunlight hitting Earth had dropped by over 10% in thirty years. This happened even as the world was getting warmer. Other researchers saw similar declines in places like Estonia, Germany, and Israel.

Most of these particles come from human activities. Before the Industrial Revolution, most sulfates came from volcanoes or ocean plankton. In 1990, it was estimated that volcanoes released 10 million tons of sulfur per year. However, human emissions from burning coal and fuel grew very fast. By 1980, humans were releasing about 80 million tons of sulfur each year. Another particle is black carbon, which people often call soot.

Since the 1990s, the trend has started to reverse. This is called global brightening. Many countries passed laws like the Clean Air Act in the United States. These laws helped reduce air pollution significantly. Because there is less pollution, more sunlight is reaching the Earth again. This change can make global warming happen even faster. Scientists use climate models to study how this will affect our future weather.

Global dimming refers to a measurable decline in the amount of solar radiation reaching the Earth's surface.

This phenomenon is primarily caused by atmospheric particulate matter, specifically aerosols. These tiny particles act as a filter between the sun and the planet. By blocking or reflecting sunlight, these aerosols can influence the Earth's temperature and weather patterns. Understanding this process is vital because it has historically masked the full extent of global warming.

The mechanism of dimming works in two distinct ways. First, sulfate aerosols act like microscopic mirrors. They reflect incoming visible light and infrared radiation directly back into space before it reaches the ground. Second, these particles serve as cloud condensation nuclei. This means water droplets in the atmosphere coalesce, or gather, around these tiny particles. When pollution levels increase, clouds form with a much larger number of smaller droplets. These clouds become more reflective, which further prevents sunlight from reaching the surface.

Scientists categorize these aerosols into different types based on their chemical makeup and effects. Sulfate aerosols are the primary driver of dimming. They are highly reflective and have a cooling effect on the atmosphere. Another significant type is black carbon, which is commonly known as soot. Black carbon is formed by the incomplete combustion of fossil fuels, wood, and plant matter. While sulfates reflect light, black carbon can absorb solar energy. This can darken clouds and change how they interact with sunlight.

Researchers began observing these trends in the 1950s through systematic solar irradiance measurements. In the 1980s, geography researcher Atsumu Ohmura discovered a significant decline in solar radiation. He found that sunlight striking the Earth had dropped by more than 10% over the previous three decades. This was a surprising discovery because global temperatures were rising during that same period. Later studies in the 1990s confirmed these declines in regions such as Estonia, Germany, and Israel.

The shift from natural to human-caused pollution is a major factor in this history. Before the Industrial Revolution, sulfate aerosols came mainly from volcanic activity and oceanic plankton. In 1990, estimates placed volcanic sulfur emissions at 10 million tons per year. However, human emissions from burning sulfur-containing fuels like coal grew rapidly. By 1980, human-caused emissions reached approximately 80 million tons per year. In industrialized parts of Europe and North America, these human emissions became ten times greater than natural sources.

Global dimming has significant consequences for the Earth's water cycle and weather. By lowering evaporation, dimming can reduce rainfall in specific regions. It may have contributed to the weakening of the South Asian monsoon. There is also evidence that it shifted the tropical rain belt southwards between 1950 and 1985. Such changes in precipitation can lead to severe environmental challenges, including famine. For example, record pollution in the Northern Hemisphere may have exacerbated the 1984 Ethiopian famine.

Since the 1990s, many nations have implemented laws to reduce air pollution. The United States passed the Clean Air Act, which helped total emissions of principal pollutants drop by 53% between 1970 and 2005. Europe also adopted measures like the 1985 Helsinki Protocol to reduce sulfur. These actions have led to a trend called global brightening, where more sunlight reaches the surface. While this improves air quality and saves billions in healthcare costs, it also accelerates global warming. As the masking effect of aerosols declines, the warming of the planet becomes more apparent.