MELISSA BLOCK, HOST:
A new and troubling milestone - record carbon dioxide levels were reported globally in March. Climate scientists say the global CO2 concentration is at a high that hasn't been seen for millions of years. These new measurements come from the National Oceanic and Atmospheric Administration. Pieter Tans is the chief greenhouse gas scientist with NOAA, and he joins me now. Welcome to the program.
PIETER TANS: Thank you. I'm happy to talk to you.
BLOCK: And explain the significance of this number. We've now crossed the 400 parts per million threshold for CO2. Is that more than a symbolic number?
TANS: Well, it's a nice milestone. It's a round number. Whether it's 395 or 405, you know, doesn't matter that much. What really matters is how much it has increased in a very short time - that means over the last one-and-a-half century - from about 280 parts per million to 400.
BLOCK: Now, when we say that this is a threshold that hasn't been reached for millions of years, how do you assess what carbon dioxide levels would have been in the air millions of years ago?
TANS: Well, the first is bubbles of air locked up in very ancient ice in Greenland and Antarctica, in particular. But there's additional information from sediments on the bottom of the Atlantic and the Pacific oceans that lets us conclude that CO2 was not as high as it is today. We have to go back approximately three to four-million years to have CO2 levels that high.
BLOCK: Now, wouldn't some people say though that if this same level was reached three or four million years ago, that that indicates climate change isn't man-made, this happened before?
TANS: Well, yes, climate has changed before dramatically. In most cases though, the pace of that was very, very slow. What is really special about modern times is that human activity on the geologic time scale is like an explosion.
BLOCK: Let's talk a bit about the methodology here and how NOAA goes about sampling carbon dioxide levels around the world. How do you do it?
TANS: For many decades, we've run what we call Global Greenhouse Gas Reference Network in a series of sites - and people at these sites who take regular air samples in glass flasks. Typically, these are oceanic sites, either on coasts or islands. We have a measurement site at the South Pole. We have a site on Christmas Island in the middle of the Pacific Ocean. So we are really trying the best we can possibly do creating a record that will be able to stand the test of time.
BLOCK: How troubling is this new record number of carbon dioxide concentration in the atmosphere - 400 parts per million?
TANS: What troubles me is that we're still headed at full speed in a direction that we should not be going. We are at the beginning of bringing about huge changes in the Earth's climate and ecosystems. The potential is there for us to make life hard, really, for future generations.
BLOCK: And what would it take to reverse the carbon dioxide concentration levels that you're seeing now?
TANS: See, that is really at the core of why we have made so little progress. The problem with CO2 in particular is that climate - forcing of climate change by CO2 depends not so much on the rate at which we are emitting it. It depends primarily on the total amount of CO2 that we've emitted since pre-industrial times.
The implication is that if we want to stop this, we have to bring the emissions back down to zero. That is a very tall order. I mean, I can understand why there is reluctance to take this on seriously. Of course, it's still in our power to make it much worse. So if we can't stop emitting at a very aggressive pace, the risks for future generations are much larger.
BLOCK: I've been talking with Pieter Tans. He's lead scientist with the Global Greenhouse Gas Reference Network. It's part of the National Oceanic and Atmospheric Administration. Pieter Tans, thanks very much.
TANS: It's my pleasure. Transcript provided by NPR, Copyright NPR.