 Species evolve through the process of natural selection. The members of the species that are best adapted to threats survive to pass on their genes to the next generation. The problem is that this process is a slow one. Humans are causing the climate to change rapidly, in a matter of decades, while big evolutionary changes generally take thousands of years. In Earth's history, there have been five catastrophic events where most species weren't able to adapt fast enough to avoid extinction. These are called mass extinction events. In most cases, these events were triggered by huge volcanic eruptions. Those eruptions pumped loads of particles and carbon dioxide into the atmosphere. The particles blocked sunlight, causing a period of sharp cooling. The carbon dioxide increased the greenhouse effect and caused long-term warming. Most species were unable to adapt to these big climate changes. The first mass extinction event happened 445 million years ago, when 86% of species went extinct at the end of the Ordovician period. Scientists think this mass extinction event was caused by an intense ice age, caused by unusual volcanic events, followed by a warm period one million years later. 360 million years ago, 75% of species went extinct at the end of the Devonian period. Scientists think that this mass extinction event was also due to a series of relatively rapid environmental and climatic changes. 250 million years ago, about 85% of species went extinct at the end of the Permian period. This mass extinction event is also known as the Great Dying, because nearly all marine species went extinct. Scientists think that this event was caused by a massive volcanic eruption, covering much of modern-day Siberia. The sulfur ash pumped into the atmosphere by this eruption, caused global cooling by blocking sunlight, and created acid rain. The carbon dioxide released into the atmosphere from the eruption later caused global warming. Trapped methane, another greenhouse gas, may have been released from the warming oceans, causing even further global warming. Do 100 million years ago, 80% of species went extinct at the end of the Triassic period. Scientists think that this mass extinction event may also have been caused by a series of large volcanic eruptions. 65 million years ago, 76% of species went extinct at the end of the Cretaceous period. This is when dinosaurs went extinct. This mass extinction event was caused by a combination of volcanic eruptions and a large meteor impact. Scientists are concerned that we may now be entering the Earth's sixth mass extinction. Based on the fossil record, it's normal for about 40 species to go extinct per year. Over the past 1,000 years, the average extinction rate has been 24 times as large as that natural rate. Over the past 500 years, extinctions are happening at least as fast as the rate that triggered the previous five mass extinction events. The good news is that mass extinction events usually happen slowly, over hundreds of thousands to millions of years. However, if we lose all currently threatened species, we'll be on course for a new mass extinction event in just over 500 years. There's a popular myth that humans and other species will simply be able to adapt to the Earth's changing climate, so we have nothing to worry about. This is the fallacy of jumping to conclusions. Unfortunately, just because adaptation exists, it doesn't mean that species can adjust to any new situation. There have been five events over the past 450 million years during which more than three quarters of the species on Earth went extinct. Species are good at adapting to change in their surroundings, but there's a limit to how fast they can evolve. Humans are changing the Earth's climate so fast that species are already struggling to keep up. We're changing it as fast or even faster than during previous mass extinction events. If we continue on our current path of warming the planet by burning lots of fossil fuels, scientists anticipate that more than 40% of species could be at risk of extinction by the end of the century. The bad news is that if we continue with business as usual reliance on fossil fuels, we're headed on a path towards the Earth's sixth mass extinction. If we fail to change our course, it will take millions of years for the planet to recover from the human-caused mass extinction events. The good news is that we're still relatively early along in the process. Although it will be a difficult task, there's still time to change course and prevent a huge loss in the Earth's biodiversity.