 The more global warming humans cause, the greater the chance that we'll trigger some really damaging consequences. Scientists don't know at exactly what point various impacts will be triggered. However, based on past climate changes and what we've seen in recent years, they can estimate what we'll see in the coming decades. So far, the Earth's average surface temperature has warmed almost one degree Celsius since the Industrial Revolution. Some significant adverse impacts are expected by the time we reach 1.5 degrees Celsius surface warming above pre-industrial temperatures. For example, that amount of global warming will worsen the problem of widespread coral mortality that we're already beginning to see. Hundreds of millions of people will be at risk of increased water stress and more damage from droughts, heat waves, and floods. There will continue to be increased species extinction rates. However, by and large, these are impacts that we should be able to adapt to at a cost but without disastrous consequences. Once we surpass 2 degrees Celsius, those impacts will become even worse and new impacts will be triggered. Coastal flooding will impact millions of people, coral bleaching will be widespread and exacerbated by ocean acidification. Most coral reefs may not survive. Global food crop production will decline with the possibility of major famines. Sea levels arise by close to 1 meter by 2100. Up to 30% of global species will be at risk of extinction. 2 degrees Celsius is used in international climate negotiations as the danger limits. It's a guardrail we don't want to pass because it represents such dangerous potential consequences. At 3 to 4 degrees Celsius warming, corals are basically toasts. The damage to aquatic ecosystems will deplete our fisheries too. If global warming reaches this level, 40 to 70% of global species will be at risk of extinction. We'll be far down the path toward the Earth's 6 baths extinction at this point. Water retreats will threaten water supplies in Central Asia and South America. The possibility of significant releases of carbon dioxide and methane from ocean hydrates and permafrost could amplify global warming even further beyond our control. At this level of warming, sea level rise of 1 meter or more would be expected by 2100 and much more in the following centuries. The destabilization of the Greenland and West Antarctic ice sheets will become a major risk. If these ice sheets collapse into the ocean, it would cause much more sea level rise and flooding of coastal communities and farmland. Many of these impacts will lead to societal problems. For example, food and water scarcity and flooding can lead to economic damages, mass migrations, and violent conflicts. Once people move beyond denying the reality that the planet is warming and humans are causing it, the next stage of denial is belief that climate change isn't bad. We'll often cherry pick a few beneficial climate change impacts, ignoring the many dangerous and damaging consequences that I just discussed. There is uncertainty about just how soon these damaging climate impacts will hit. However, if we continue on our current path pumping billions of tons of carbon pollution into the atmosphere every year, we know we'll trigger these harmful climate consequences eventually. Climate change is ultimately a risk management issue. It's very much like smoking in that respect. The more cigarettes you smoke, the more you increase your risk of getting cancer. Most people value their health enough that they mitigate that risk by choosing not to smoke. We face the same choice with climate change. The more carbon pollution we pump into the atmosphere, the more we increase the risk of triggering dangerous and damaging climate changes. For example, coral reef loss, widespread species extinctions, and more damaging extreme weather, and the loss of coastal property sea level rise. We only have one earth and one climate. Although it makes for good science fiction, we're unlikely to have interstellar travel available within the next century. We won't be able to solve the problem by ditching the earth and moving to a new planet. To mitigate the risk associated with climate change, the solution is to consume less fossil fuels and pump less carbon pollution into the atmosphere. Just as with smoking, the more we reduce the consumption that is causing the problem, the better our chances of avoiding its dangerous consequences.