David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.
The weather gets hotter, and the weather gets colder; it fluctuates day-by-day, year-by-year, and from place to place. But what do we see when we look on a large scale, and in the long term? That is climate: variations in the average weather in a wide area, over a long period of time.
Today we're seeing unprecedented climate change, and we have strong evidence that suggests human greenhouse gas emissions are the cause. Greenhouses gases are gases that due to their molecular make-up are able to trap heat in the atmosphere, acting like a blanket on the earth. The more of these gases we put into the atmosphere, the warmer the earth gets. The most significant of these, Carbon Dioxide, has reached 400 parts per million (ppm) from 280 ppm at the start of the Industrial Revolution. It was as low as 350 ppm as recently as 1990.
But to really understand that today's global warming is unprecedented and unnatural, we need to know what is natural.
The earth naturally warms and cools over hundreds of thousands of years. We call the cool periods ice ages, times where the earth's ice caps expand further from the poles. We have to be able to look at the earth's natural cycles and subtract those cycles from what we're seeing. When we do that we find that the Earth's temperature is indeed warming at a staggering rate, far faster than would be expected naturally. In this lesson, we're going to go through some of the natural cycles the Earth undergoes, and the causes of those cycles.
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The way we often picture the solar system is oversimplified. We tend to imagine that everything is neat and tidy; that the Earth orbits the Sun in a perfect circle, that the Earth is a perfect sphere, that the Earth tilt never changes, and that the orbits and rotations of everything stays the same. But the truth is a bit more complex than that.
The Earth's orbit around the Sun changes over time from nearly circular, to an elliptical shape. It goes back and forth between these two shapes in a cycle that repeats roughly every 100,000 years.
The Earth's tilt also changes. The Earth moves back and forth between a tilt of 22.1 degrees and 24.5 degrees every 41,000 years. But not only does the amount of tilt change, but the direction of the tilt also changes. The Earth doesn't spin in a neat, balanced way. It's more like a spinning top that's starting to slow down and wobble. The Earth wobbles on its axis, making a circle every 26,000 years.
Together, the changing orbit, tilt, and wobble are called Milankovitch cycles, which are one of the causes of ice ages. How the Earth is tilted and how long the planet is far away from the sun (during the elliptical orbits) affects the areas and strength of the sun's exposure, encouraging or discouraging the formation of ice. Right now, ice ages are coming and going every 100,000 years, but at other times in history the cycles were shorter and more of a big deal, down to 26,000 years.
Other factors that affect ice ages include ocean and atmosphere circulation, volcanoes erupting, and carbon dioxide levels in the atmosphere.
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So that might explain long term, big ice ages. But we also have smaller climate variations. One of these is the way the Sun's output changes. This happens on an 11-year cycle. Scientists have spent a lot of time trying to research how it affects the climate, but so far it looks like the effect is very small. It makes the Earth a little cooler and warmer, but not a huge difference.
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A more significant climate cycle on the Earth is the El Niño and La Niña cycles. El Niño is a time when the east Pacific and west coast of South America experiences unusually strong storms brought on by warm sea and air currents moving east from the western Pacific. They happen every 3-6 years. This has all kinds of knock-on effects for weather around the world. La Niña is the opposite - a time when the east Pacific and west coast of South America experiences a dry spell due to warm sea and air currents moving west. In between the two, conditions are normal.
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We unfortunately don't know for sure what causes the changes in currents which drive the El Niño and La Niña cycle - it appears to be a complex mix of factors - but it is an area of continuous study.
Although humans have impacted the climate, the climate also naturally changes over the years. To understand how much humans are impacting the climate, we first have to subtract the natural cycles and patterns. Those natural cycles include Milankovich cycles, sun cycles, El Niño and La Niña and many others.
Milankovitch cycles are 100,000 year climate cycles that control the larger ice-ages, and happen because of the changes in the Earth's orbit, tilt and the wobble of the Earth on its axis.
Sun cycles are 11-year cycles in the Sun's output that have only minor impacts on the climate. El Niño and La Niña are times of high rainfall (El Niño) and low rainfall (La Niña) in the east Pacific and South America that happen every 3-6 years. This is caused by sea and air currents changing over time, though we don't know exactly why those changes happen.
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