For most of Earth’s 4.5 billion-year history, the climate was shaped by natural forces.
Shifts in ocean patterns and the natural release of carbon dioxide from peatlands contributed to cyclical climate patterns. With the data we have today, scientists believe that the planet went through a series of long ice ages over the past 650,000 years.
It wasn’t until about 100,000 years ago that modern humans began to successfully populate and alter the landscape to meet our needs.
Today, Earth has nearly 8 billion human inhabitants. As we live in an unprecedented time in history when humans have dominated the planet, how much of a role does nature still play part in influencing our climate?
In this article, we explore the role of natural variability in shaping our climate.
What is natural variability?
You’ve probably experienced a summer that felt cooler than normal or a winter that was so warm that even artificial snow melted.
These colder than average periods and warmer than average periods are a result of natural variability, which is an occasional change of weather patterns from year to year or every couple of years. It’s highly noticeable and shapes the way we experience the seasons.
Natural variability is caused by a combination of natural complex occurrences from internal instabilities in the atmosphere, which create everything from cyclones to cold polar outbreaks. These anomalies cause unusual increases and decreases in temperature despite the temperature balance afforded by incoming solar radiation and the heat that’s absorbed by greenhouse gases.
Natural variability is also influenced by interactions between the ocean and the atmosphere. Weather events such as El Niño, which occur in the tropical Pacific Ocean, can happen over months and decades with serious impacts on the seasonality and climate of a particular region.
These events change the distribution of ocean surface temperatures. Warmer temperatures can cause increased evaporation and faster winds. As the air circulating above the waters gets heavier with moisture, hurricanes and other tropical storms grow stronger and more frequent.
Rainfall patterns also shift along with these changes. Some regions may receive more rain while others become more prone to drought.
What does climate variability mean?
Climate variability can be thought of as changes in climate patterns over decades and centuries. It encompasses all of the weather patterns occurring in a particular area and represents the average of all the weather events.
It’s believed that variations in the climate occur in cycles. Over the last 800,000 years, our planet has experienced a series of ice ages broken up by periods of warmer interglacial activity. The last ice age ended about 20,000 years ago and since then, the average global temperature has risen from 3 degrees celsius to 8 degrees celsius.
Climate scientists attribute the cyclical changes in our climate to factors that are mostly internal to the climate system. These include changes in greenhouse gas concentrations and ocean patterns. Greenhouse gases influence how much heat our atmosphere traps while the ocean helps translate the changes in temperature into worldwide weather patterns.
They also point to external factors such as changes in the sun, the Earth’s orbit, volcanic eruptions, and other interactions between snow, sea ice, and glaciers that influence the climate. Together with greenhouse gases and the ocean, these factors make up the forces that drive the oscillations between the ice ages and the warmer interglacial periods.
What are some factors in natural climate variability?
Natural climate variability is primarily determined by ocean patterns, the amount of incoming solar radiation, and changes in Earth’s orbit. These factors shape climate patterns over hundreds of years and make our planet’s ice ages possible.
The ocean covers 70 percent of our planet’s surface. It has an immense capacity to absorb heat and release moisture into the atmosphere, which makes it the largest contributor to the alteration of atmospheric circulation.
Slight changes in ocean circulation, as well as ocean temperature, can cause deviations in the distribution of clouds and water vapor. These may influence the rainfall patterns in different zones of the Pacific Ocean.
In the West Pacific Monsoon (WPM) zone, for example, large differences in temperature between the land and ocean can affect the severity of the monsoon rains. Conditions may suddenly go from very dry to very wet. The WPM normally travels north to mainland Asia between June and September and then to Australia between November and February.
Every 11 years, the Sun’s magnetic fields flip from north to south and back. In between the flips, radiation from the Sun ebbs and flows by up to 0.15 percent. Scientists believe that over decades or centuries, the fluctuations in solar radiation reach Earth and cause its temperature to increase by about 0.1 degrees celsius.
According to models developed at NASA’s Goddard Institute for Space Studies, incoming solar radiation has caused Earth’s average temperature to increase by 0.15 degrees Celsius over the past century.
Though it’s still unclear whether solar radiation will have more of an impact on our climate in the future, scientists are concerned that melting glaciers and our expanding ocean will absorb more of the sun’s energy and increase climate variability.
Changes in the Earth’s orbit may also affect the amount of solar radiation the planet receives. Some scientific theories assert that the Earth travels in three different elliptical cycles around the Sun. As the Earth completes each separate orbit, the tilt of its axis, as well as its distance from the sun, can affect which regions of the planet receive solar radiation and how much.
These cyclical orbital movements, known as Milankovitch Cycles, are believed to be responsible for starting and ending ice ages. When low amounts of solar radiation hit Earth’s northern hemisphere over thousands of years, the great continental ice sheets grew, cooling down the Earth’s climate. The scientist who came up with this theory, Milutin Milankovitch, calculated that ice ages occur at 41,000 year-intervals.
Nonhuman examples of natural climate variability
Natural climate variability can be categorized as external or internal to the climate system. Ocean patterns such as El Niño are internal to the climate as they influence atmospheric circulation and rainfall. Other natural activities such as volcanic eruptions are considered external to the climate as they don’t regularly influence the climate.
Here’s a look at some nonhuman examples of natural climate variability.
The El Niño Southern Oscillation (ENSO) is a natural variation caused by changing patterns in ocean circulation. It creates cyclical patterns of climate variability in the Pacific Ocean that lasts 6 to 18 months on average.
ENSO occurs when the surface of the Pacific Ocean becomes warmer than normal. The change in temperature affects the strength of rainfall, cyclone activity, and ocean currents.
During El Niño events, the southern United States experiences a wetter winter than usual while the northern part of the country goes through a dry season. It also drives climatic variability over the Brazilian Amazon, where it’s understood to cause severe droughts in large parts of the rainforest. Strong ENSO events
Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is a climatic event that occurs in the Pacific Ocean for periods of up to 30 years. Though it’s not yet known what factors cause the PDO, some researchers have hypothesized that it could be a combination of the effects of both El Nino and La Nina together with changes in atmospheric pressure over the northern Pacific.
Changes in the PDO can cause frequent hurricane activity, droughts and flooding in East Asia and the western coasts of North and South America, and variations in global land temperatures.
Large-scale volcanic eruptions are also known to disrupt atmospheric conditions. Enormous amounts of ash ejected high into the atmosphere can stay there for months or years, reflecting solar radiation back into space. With very little incoming sunlight, the Earth experiences a drop in global surface temperatures.
When Mt. Pinatubo erupted in 1991, it released 15 million tons of sulfur dioxide into the stratosphere. Over the next two years, scientists observed a 0.6 degrees Celsius drop in global average temperatures linked to the eruption.
What’s the difference between natural variability and manmade climate change?
The latest climate research indicates that natural variations in the Earth’s climate have driven a relatively consistent cycle of ice ages and warm interglacial periods over thousands of years. Global average temperatures rise gradually as climate factors including greenhouse gases, ocean circulation, and the Earth’s orbit combine create shifts in the climate.
But since the 1850s, our planet’s average surface air temperature has risen by 1 degree Celsius. Every decade, the Earth warms by an additional 0.2 degrees celsius which is also causing the oceans to warm.
Studies into this phenomenon concluded that the rate at which the Earth was warming seemed unnatural. The rapid warming was inconsistent with the scientific understanding of natural climate variability caused by solar output, ocean currents, and volcanic activity.
Volcanic activity was high in the twentieth century and solar radiation was low, which should have resulted in a slight cooling of temperatures in the latter half of the twentieth century. Also, the stratosphere was discovered to be cool, which meant that solar radiation could not be responsible for the increase in temperatures. If that was the case, both the lower atmosphere and stratosphere would have warmed.
It was determined that the warming could only be the result of human industrialization – unprecedented amounts of greenhouse gases from the burning of fossil fuels were trapping heat in our atmosphere and warming the Earth’s climate over the past 100 years.
According to the Intergovernmental Panel on Climate Change (IPCC), concentrations of greenhouse gases in the atmosphere are currently at their highest in 800,000 years. Without urgent climate action, the planet could warm by 4 degrees Celsius above pre-industrial levels by 2100.
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