In the last 650,000 years of Earth's history, there have been seven climate changes- most of which attributed to slight variations of the planet's orbit. But today we are going through Earth's first anthropogenic one.
Climate change is a global or regional alteration of Earth's average temperatures brought from weather patterns over a period of time, that affect all of its beings because they have all evolved or adapted to the averages they have been used to for thousands of years. Our planet has historically experienced climate change before from natural cycles or from foreign objects colliding with the Earth's surface. Sometimes climate change is from global cooling, like an ice age or Fimbulvetr, and sometimes it's from global warming, like what the Earth is currently experiencing.
The oldest recording of temperature was practiced at the Central England Temperature record in 1659 and we began recording the temperature globally, and in the same way, in 1873. Not a very long period of time from then and now to really determine something so drastic as climatic change. However, there are other ways climate scientists look for differences throughout history. One of the easiest and most obvious ways is by studying tree rings from trees that began growing prior to the 1800's. Tree rings reveal a history of their life as they've grown; wider rings in warm, wet seasons and thinner in cold and dry weather, and consistency of one or the other reveals a trend. And another way of determining global warming prior to the mid-19th century, is by studying items you would not normally think to look at: paintings. Wild landscapes have been painted by artists for centuries and these can reveal snow-capped mountains that no longer have snow, glaciers larger than anyone has ever seen them before, or water levels reaching places that have long been covered in foliage and tall trees.
why it's happening
97% of scientists across the world believe that our current global warming is 95% probable to be caused by anthropogenic (human-induced) means. Due to our current warming, glaciers are melting and sliding to reveal ice that hasn't been so close to the surface in hundreds of thousands of years. Researchers took advantage of this and were able to access air bubbles trapped in Antarctic ice to test its carbon dioxide levels from when that ice first formed, about a million years ago. They found that the amount of carbon dioxide, a natural gas in our atmosphere never rose above 300 parts per million (ppm). Comparing it to air bubbles from other ice formations, they discovered that, outside of miniature climate changes, this amount fluctuated between 280-300 ppm from a million years ago, all the way up to 150 years ago. The carbon dioxide levels we have today are over 400 ppm in the atmosphere, which means that our ancestors have never felt the temperatures we are currently experiencing.
There are four main greenhouse gases (gases that trap heat that thus, act like a greenhouse) that humans emit into the atmosphere: carbon dioxide, methane, nitrous oxide, and fluorinated gases; collectively increasing by 35% in just twenty years. Carbon dioxide is the highest amount of gas released by humans, but methane traps up to 30 times more heat than carbon dioxide. These four gases are released from combustion of fossil fuels like oil, coal, and natural gas that formed millions of years ago, as well as from other human-controlled sources. The Earth is warmed by our sun and the Earth releases this heat back into the atmosphere; some of which stays, and some of it released out. However, as more and more of these greenhouse gases are released by us, the thicker this barrier becomes and the more this heat is trapped to remain on Earth. There is one additoinal greenhouse gas that is indirectly caused by humans, and that is water vapor. As the warmer our planet gets from burning fossil fuels, the more water is evaporated which increases the levels of water vapor into the atmosphere. The water vapor then absorbs thermal energy radiated from the Earth, as it normally does, which then also increases atmospheric temperatures; creating more water vapor and more absorption, and so on.
Some skeptics believe that global warming is caused by the change in the sun or from sunspots. However, since 1750, the sun's energy has remained constant or changed only slightly. Also, if it was caused by the sun, we would see a rise in temperatures throughout all the layers of the atmosphere. But we are only seeing a rise in the lower layers, closest to the Earth, and a cooling of temperature in the upper layers, closest to the sun.
what's being done
The number one priority in reversing our climate change is by replacing our fossil fuel-powered lifestyles with renewable energy, instead. Other than relying on alternative energy, mitigating deforestation would also greatly reduce global warming since these trees would otherwise use carbon dioxide to generate photosynthesis and create oxygen for the Earth's beings. When plants and trees, however, conduct photosynthesis, the process can be painstakingly slow. German researchers are in the testing process of creating synthesized enzymes called RuBisCO that can convert carbon dioxide into organic compounds, known as carbon fixation. This is what normally happens during the Calvin cycle of photosynthesis, but by creating our own, we can make it absorb more and faster than plants or trees could ever dream of (if...they dreamed).
When humans collectively work to lower their contributed greenhouse gas emissions, this prolongs the warming which can allow scientists to develop better ideas and technologies to reduce the amount in our atmosphere back to what it was in a pre-Industrial time. Many of these efforts are minute like turning their houses into more sustainable eco-homes, using less water, eating less meat, carpooling or driving an electric car, changing to energy-efficient appliances, et cetera. But if enough people do enough to change their lifestyles, then together we can help to stop climate change from happening and create a better environment for our children and the children of all of the Earth's beings.