100 percent of global warming over the past century has been caused by humans. In 2013, the Intergovernmental Panel on Climate Change Fifth Assessment Report stated a clear expert consensus that: “It is extremely likely [defined as 95-100% certainty] that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic [human-caused] increase in greenhouse gas concentrations and other anthropogenic forcings together.”
Then in 2014, research in the journal Climate Risk Management using rigorous statistical techniques revealed an objective link between global temperature increases and human activity, with a probability exceeding 99.999 percent. In fact, according to a Skeptical Science review of studies on human and natural contributions to global warming:
“Most studies showed that recent natural contributions have been in the cooling direction, thereby masking part of the human contribution and in some cases causing it to exceed 100% of the total warming.”
The overwhelming majority of scientists—about 97 percent—agree not only that climate change is happening, but that it is caused by humans. Nevertheless, most people don't agree: they tend to disbelieve these kinds of statistics, or see climate change denial as an equally valid, “alternative” point of view.
Despite clear evidence that global warming is caused by humans, many people believe natural processes are playing a major role: only 43 percent of people in the U.K., 49 percent of Germans, 34 percent of Norwegians, and 55 percent of the French believe that climate change is mostly or completely caused by humans. Even fewer people—only about one-third of all people in these four countries—believe that more than 80 percent of scientists agree that climate change is a real, human-caused phenomenon.
In the U.S., Pew data shows that about 48 percent of all adults believe climate change is caused by humans, but about 31 percent believe that global warming is the result of natural causes. A full 20 percent believe that it doesn't exist at all. Only about 40 percent of Americans expect global warming will have harmful effects on wildlife, weather patterns, and shorelines. When it comes to that figure citing "almost all" climate scientists agreeing that global warming is caused by humans, only 27 percent of Americans believe that's true. As for everyone else? 35 percent saying “more than half,” of global warming is caused by humans, 20 percent saying “about half,” is and 15 percent believe “fewer than half or almost none.”
Although these differences in opinion exist along political lines in the U.S. in particular, it is critically important to drive conversations about climate change. Forcing policy changes in the current political climate isn't easy, and without public pressure and widespread support, it's near impossible. This is why helping people understand the facts about climate change is so important. Species are already dying, and extreme, climate change-induced weather is already killing people. This is terrible news, but it does mean that the facts will be harder and harder to ignore. Maybe then the conversations will be easier to have.