TODAY   |  May 13, 2013

Scientists issue dire warning about global warming

The amount of carbon dioxide in the atmosphere has surpassed a threshold that scientists say signals we are running out of time to reverse the impact of global warming. NBC’s Anne Thompson reports.

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>>> there are growing concerns over the environment in morning. carbon dioxide has reached levels not seen in years. anne thompson , good morning to you.

>> good morning, matt. it's just one day's measurement of the co2 levels in our atmosphere. it's a number that fluctuates throughout the year but the amount of co2 surpassed the threshold that scientists say signals we are running out of time to reverse the impact of global warming . from the serene setting high atop hawaii's mauna loa volcano a danger sign, the highest carbon dioxide levels in human history , 400 parts per million .

>> the rate of change is accelerating, that's something all of us need to be concerned about.

>> reporter: the amount of carbon dioxide has risen dramatically since the industrial revolution from 280 parts per million to the recent 400 parts per million milestone. man made carbon dioxide is produced when we use coal for electricity or oil and gas to get around. co2 traps heat in the atmosphere. too much and it warms the earth, changing the climate causing ice caps and ice sheets to melt, sea levels to rise, and how much is too much? many scientists say we are rapidly approaching the 450 parts per million that could push climate change beyond the point of no return but some disagree saying those fears are wildly overblown.

>> i think probably in my lifetime we'll surpass 500 parts per million co2 but it simply should be now recognized it has no bearing on the temperature of the earth.

>> reporter: a growing number of scientists say it is not a risk worth taking.

>> do we really want to play you know chicken with the environment, when in fact we don't need to?

>> adding to the problem, carbon dioxide stays in the atmosphere for decades which means that reversing the trend will not be an easy task.

>> you said we haven't seen levels of carbon dioxide in the atmosphere like this for millions of years so just give me a sense, and i'm not suggesting you were covering it back then.

>> there are days when i feel like i was. hey!

>> give me a sense what was happening with the planet the last time the levels got this high.

>> it was actually 3 million years ago which is about 1 million years before i was born but the last time this happened, the sea levels were 60 to 80 feet higher than they are today so just imagine. most of the earth was covered with water, much more, three-quarters of the earth is covered with water today so imagine a completely different planet and it was long before humans were walking the face of the earth.

>> anne thompson thank you very