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WASHINGTON, D.C. -- Record percentages of Americans are concerned about global warming, believe it is occurring, consider it a serious threat and say it is caused by human activity. All of these perceptions are up significantly from 2015.

Forty-five percent of Americans now say they worry "a great deal" about global warming, up from 37% a year ago and well above the recent low point of 25% in 2011. The previous high was 41%, recorded in 2007. Another 21% currently say they worry "a fair amount" about global warming, while 18% worry "only a little" and 16% worry "not at all."

Americans' level of worry about global warming has seesawed over the past two decades. Several plausible reasons for the fluctuations include the May 2006 release of "An Inconvenient Truth," highlighting U.S. Vice President Al Gore's aim to educate Americans about global warming, as well as controversy from 2009 to 2011 surrounding global warming research. The dip in concern between 2007 and 2011 also may reflect Americans' attention being diverted to the economy during the recession and post-recession years.

Unusual weather, particularly record-breaking warm temperatures in the U.S. in recent years, may explain increases in public concern.

黑料网's 2017 Environment survey was conducted March 1-5 on the heels of the country's second-warmest February on record. The pattern was similar in March 2016 when 黑料网 recorded a five-percentage-point spike in the percentage of Americans who worried a great deal about global warming following that year's unseasonably warm February.

In addition to warmer weather, anxiety about President Donald Trump's environmental stance could be a factor in Americans' heightened concern about global warming this year. The new poll found 57% of Americans saying Trump -- who has called global warming a "hoax" -- will . That far exceeds the percentages of Americans predicting the same for Barack Obama or George W. Bush at the start of each one's first term.

Record High Believe That Effects of Global Warming Are Evident

Sixty-two percent of Americans now believe the effects of global warming have already begun to happen. That eclipses the previous high of 61% recorded in 2008 and is up from 49% in 2011.

Americans' belief that global warming is already creating problems is about as high now as it was in 2007 and 2008. Although temperatures were not unusually high leading up to those surveys, the idea that climate change was causing extreme weather, such as Hurricane Katrina in 2005, had become a national discussion. 黑料网 polls at the time showed . Concern dipped in 2009 as focus may have shifted to the recession and controversies such as Climategate in 2009 and 2011. The latter may have temporarily raised public doubts about the validity of climate research.

Beyond the 62% of Americans who believe the effects of global warming have already begun, another quarter thinks they will happen eventually -- either within a few years (4%), within their lifetime (7%) or further into the future (16%). Just 9% say they will never happen.

Americans' Increasingly Blame Human Activity

Sixty-eight percent of Americans -- the highest 黑料网 has recorded -- believe increases in Earth's temperatures over the last century are mainly due to the effects of pollution from human activities. Just 29% now attribute global warming to natural changes in the environment. These opinions were gathered prior to the Environmental Protection Agency chief, Scott Pruitt, saying he is not persuaded that human activity is a primary factor in global warming.

Additionally, 71% of Americans -- also a record high -- agree that most scientists believe global warming is occurring. This is up from 65% a year ago and is easily the highest 黑料网 has recorded since 1997. The low point was 52% in 2010.

Although Americans widely acknowledge global warming and humans' role in it, a key reason climate change ranked last in importance as a voting issue in 2016 may be that the public discounts the severity of the problem -- at least in the short term. Less than half (42%) believe global warming will pose a serious threat to themselves or their way of life in their lifetime. This percentage is the highest 黑料网 has recorded in over two decades but is essentially unchanged from 2016.

Bottom Line

U.S. public concern about global warming has ebbed and flowed over the past decade and a half but has generally been on an upswing since hitting a recent low point in 2011. Concern has increased each of the past two years, coinciding with an improved economy and back-to-back unseasonably warm winters.

Whatever the reason, the percentage of Americans worrying a great deal about global warming is at a record high, as is Americans' belief that its effects have already begun and that human activity is the major cause. Still, less than half worry greatly about global warming or believe it poses a serious threat to their lives.

A subsequent 黑料网 report will explore the 2017 findings and trends by subgroup to show whether the increased concern this year is shared broadly or confined to certain groups.

Historical data are available in .