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Global warming and the problems caused by human activities

We know that warming—and cooling—has happened in the pastand long before humans were around. Scientists have devised different methods to answer this question. Meteorologists and oceanographers compare the climate patterns they observe with patterns developed using sophisticated models of Earth's atmosphere and ocean. Scientists have gathered evidence and have improved their methods for teasing apart natural and human factors. Today scientists have very high confidence about human-caused global average surface temperature increase — a key climate indicator.

Direct evidence of human contribution to atmospheric CO2 Carbon dioxide concentrations. Climate Central Carbon dioxide CO2 is the main heat-trapping gas largely responsible for most of the average global warming and the problems caused by human activities over the past several decades.

The atmospheric concentration of CO2 has increased dramatically, from a pre-industrial era AD 1000 — 1750 concentration of approximately 280 parts per million ppm to today's 400 ppm. Scientists warned for years about this dangerous threshold, but with the accelerated pace of emissions the question changed from whether we would reach CO2 concentrations above 400ppm to when. The Arctic reached 400 ppm in 2012.

In 2013 the Mauna Loa Observatory in Hawaii recorded more than 400ppm. In March 2015 global averages reached this threshold, and in September 2016 the world reached a point of no-return: CO2 concentration levels are unlikely to dip below 400 ppm again. While the concentration of carbon has increased, the carbon originating from natural sources has decreased. Scientists can tease apart how much CO2 comes from natural sources, and how much comes from combusted fossil fuel sources.

How Do We Know that Humans Are the Major Cause of Global Warming?

This information tells scientists that fossil fuel emissions are the largest contributor of CO2 concentrations since the pre-industrial era. Volcanic events and some types of human-made pollution, both of which inject sunlight-reflecting aerosols i. Human activity drives climate change. History of Climate Drivers: Volcanic eruptions account for the cooling spikes seen in the graph in 1883 and 1991.

IPCC AR 5 Among natural drivers, a large volcanic eruption can have a sharp cooling influence as it spews tiny particles high into the stratosphere the layer of the atmosphere above the troposphere where weather typically occurs. The massive explosions from Krakatoa Indonesia in 1883 and Mount Pinatubo Philippines in 1991, for example, can be seen as the two largest downward spikes in the volcanic data depicted in the figure to the right.

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These particles prevented the full energy of the sun from reaching the surface of Earth and created a cooling trend for several years. Fossil fuel burning by humans emits tiny particles in addition to releasing CO2 in the atmosphere. Some particles reflect sunlight back to space aerosolssimilar to the volcanic particles, having a cooling effect. Other particles such as soot black carbon absorb the sunlight and drive temperature rise, leading to local warming of the atmosphere level where the soot particles circulate.

Very likely, there would have been even more warming in the past 60 years if it were not for these human-made and natural tiny particles. Much as the Global warming and the problems caused by human activities Force develops computer programs to simulate aircraft flight under different conditions, climate scientists develop computer programs to simulate global climate changes under different conditions. These programs use our knowledge of physical, chemical, and biological processes that occur within Earth's atmosphere and oceans and on its land surfaces.

Mathematical models allow scientists to simulate the behavior of complex systems like climate and explore how these systems respond to natural and human factors. When models take into account both natural and human drivers, they better reflect the observed changes in temperature. IPCC AR5 For a computer model to accurately project the future climate, scientists must first ensure that it accurately reproduces already observed temperature changes i. Scientists use climate computer models to compare these observed changes with natural climate drivers and human climate drivers.

When climate models include only recorded natural climate drivers, the models cannot accurately reproduce the observed warming of the past half century. When the models also include human-induced climate drivers, then they accurately capture recent temperature increases in the atmosphere and in the oceans. And when all the natural and human-induced climate drivers are compared to one another, the dramatic accumulation of carbon from human sources is by far the largest climate change driver over the past half century.

Confidence in attribution per type of event.

3. Are human activities causing climate change?

The National Academies of Sciences, Engineering and Medicine Over the years, the models of attribution science have become more sophisticated, and, in addition to attributing global warming to human caused emissions, they are also able to determine the contributions of global warming to extreme events like floods, heat waves, storms, etc.

The report showed that scientists can evaluate the effect of climate change on a single extreme event--how human-caused emissions can increase the likelihood of that event to happen. Although attribution science is clearer for some types of events than for others, it is an important step to provide predictive forecasts of extreme events at longer lead times, reducing risks and improving preparedness. Solutions within our reach We are the cause, we are the solution.

Knowing that human activities are the main driver of global warming helps us understand how and why our climate is changing, and it clearly defines the problem as one that is within our power to address.

  • Scientists can tease apart how much CO2 comes from natural sources, and how much comes from combusted fossil fuel sources;
  • The main drivers that have acted over the last century are:

We cannot avoid some level of warming caused by the heat-trapping emissions already present in the atmosphere, some of which such as carbon dioxide and nitrous oxide last for 100 years or more. But, with aggressive measures to reduce emissions and adapt to those changes we cannot avoid, we have a small window to avoid truly dangerous warming and provide future generations with a sustainable world. We can reach that goal through immediate and sustained action to reduce our heat-trapping emissions like adopting technologies that increase energy efficiency, expanding our use of renewable energy, and slowing deforestation among other solutions.

Human activities are impacting the climate system.

August 1, 2017 We Need Your Support to Make Change Happen We can reduce global warming emissions and ensure communities have the resources they need to withstand the effects of climate change—but not without you. Your generous support helps develop science-based solutions for a healthy, safe, and sustainable future.