( CNN ) -- Science has brought us incredible gifts : life-saving vaccines and clean water , air travel and instant communication . Modern life is built upon the knowledge that the scientific community has gathered and that society -- markets , governments , workers -- put to use . We have our ancestors to thank . But will our descendants thank us ? Have we put to good use the knowledge we are gaining today to help those who will come after us ? That question is now before one of the highest courts of the United States . As a scientist who has spent the past half-century documenting how mankind is fundamentally altering our climate , I fear that unless the courts understand the threat and require the government to produce a plan of action , the answer of history could be damning . In a new study , I and my co-authors make indisputably clear what the world 's scientists have increasingly warned : Our climate is changing , and the impacts are growing . The changes harm humans and threaten other life on the planet . Our study , published in the prestigious peer-reviewed science journal PLOS-ONE , was written in support of a lawsuit against the federal government . The plaintiffs are young people , those to whom we are handing an increasingly warmer and destabilized planet . They argue that they have a constitutional right to a safe climate , that they have a right to receive from us a planet that supports all life , just as our forebears gave us . It is correctly a legal argument , but it relates to a fundamental moral question . We know without a doubt that gases we are adding to the air have caused a planetary energy imbalance and global warming , already 0.8 degrees Celsius since pre-industrial times . This warming is driving an increase in extreme weather , from heat waves and droughts to wildfires and stronger storms ( though mistakenly expecting science to instantly document links to specific events misses the forest for the trees ) . More than simply listing calamitous threats , we wanted to jump-start the discussion of how the world can take significant action . Now . First , the good news . We still have time to choose . Will we seek out and exploit ever deeper and dirtier fossil fuels or phase in cleaner carbon-free energy ? If we start aggressively reducing carbon emissions , we can stabilize our climate and avoid cataclysmic change . Our fate truly is in our hands . Our analysis shows that we must rapidly reduce carbon emissions and improve land management policies to restore billions of tons of carbon to soils and forests . We recognize that wo n't be easy . Carbon emissions will decline only if the price of fossil fuels begins to include their costs to society : their effects on human health and climate . Economic analysis shows that a rising carbon fee collected from fossil fuel companies would swiftly drive market innovations and investments in clean energy . ( Indeed , many companies are preparing for such a fee . ) Watch out for climate surprises , scientists warn . Courts can not tell the government how to reduce emissions . But they can require that the government provide a plan : How will emissions be reduced to assure that the rights of young people are protected ? There is also painfully challenging news . The widely accepted goal of limiting global temperature rise to 2 degrees Celsius would lead to continually receding shorelines and growing climate extremes . We must , instead , keep maximum warming close to 1 degree Celsius . Thus , most of the allowed carbon emissions budget has been used up . We must phase out coal emissions . And it would be foolhardy to develop unconventional fossil fuels such as tar sands . In short , our analysis finds that we must act more quickly and more aggressively than politicians and international negotiators have even considered . My co-authors are experts on biology , ecology , law , economics , public health , climate modeling , oceanography and paleoclimatology . We need broad expertise because climate change is not just a science problem . It is the great challenge of our times . Climate change warriors : It 's time to go nuclear . Climate change is altering people 's lives , right now , from the United States to Africa to the Arctic . It is as clear and present a danger as we 've ever seen . And the threat will only get worse if left unchecked . If we allow the planet 's energy imbalance to increase and the ocean to continue to warm , we will pass a point of no return . Our children and grandchildren would inherit disintegrating ice sheets , rising seas that inundate all coastal cities , increasingly violent climate extremes and extermination of countless species . We would leave future generations a far more desolate planet than the one that we were blessed to inherit . Science and the knowledge it brings is a gift only if we use it . Failing to act on our knowledge to protect future generations would be the gravest injustice of all . The opinions expressed in this commentary are solely those of James Hansen .
David Hansen : We know what to do to prevent climate catastrophe . Ending emissions from coal and stopping development of tar sands are key , he says . It 's time to establish a fee for companies that emit carbon , Hansen says . Hansen : It would be unjust to bequeath to our descendants a desolate land .