[MD] Global dimming(and all it's supported data) part 2

[Ron Kulp]

PETER COX: Since we're pumping up greenhouse gases, we actually provide
what's called a forcing on the climate system, a warming effect, which
you can measure in terms of watts-per-meter-squared, much like you could
measure watts from a light bulb. And that forces the climate to a warmer
state.

NARRATOR: Because greenhouse gases trap heat, when we add to them we
increase the heat energy trapped in the atmosphere. Today, the extra
energy trapped by manmade greenhouse gases would be enough to run a
100-watt light bulb, placed every six meters over the entire surface of
the globe, an extra 2.6 to three watts of energy for every square meter.
It's this extra energy that's driving global temperatures ever higher.

But it's now clear to the world's climate scientists that this
greenhouse warming is not the only factor at play. There's also the
cooling from global dimming. The question is, "How big an effect is it
having?"

In 2002, NASA launched the Aqua satellite. Onboard was a suite of
instruments designed to measure the effect of dimming pollutants on the
energy budget of the Earth. The observations from Aqua have enabled
climate scientists to make a rough estimate of global dimming's total
cooling effect on our planet.

JAMES HANSEN: Our estimate for the particle forcing is
minus-one-and-a-half-watts- per-meter-squared. So that would imply a
cooling of more than one degree Celsius.

NARRATOR: In other words, while the human greenhouse effect has produced
2.6 to three watts of extra energy for every square meter of the Earth,
global dimming has subtracted about 1.5 watts, so, more than half the
warming effect of our greenhouse emissions has been masked by the
cooling effect of particle pollution.

Perhaps this is why, despite a large rise in the concentration of
greenhouse gases, until recently, the temperature rise has been hard for
most of us to notice.

JAMES HANSEN: In a way, it is unfortunate that the small particles were
in the atmosphere because we would have realized much earlier that
the...how strong the greenhouse effect is, and would have had more time
to make the adjustments that are going to be necessary to slow down and
eventually stop the growth of greenhouse gases.

NARRATOR: Despite the cooling from global dimming, scientists agree that
over the past century or so, average temperatures have risen between .6
and .8 degrees Celsius, about one to one and a half degrees Fahrenheit.

The increase, small as it may seem, is very fast by the standards of
Earth history, but now we face something much faster. Ironically, if we
keep bringing particle pollutants down-with great benefits to health-but
continue pumping greenhouse gases into the atmosphere, Peter Cox
believes we could be creating the worst possible combination for global
temperatures.

PETER COX: We're going to be in a situation, unless we act, where the
cooling pollutant is dropping off while the warming pollutant is going
up. CO2 will be going up and particles will be dropping off, and that
means that we'll get an accelerated warming. We'll get a double whammy.
We'll get reducing cooling and increased heating at the same time, and
that's, that's a problem for us.

JAMES HANSEN: If the particle forcing is what we estimate, about
minus-1.5 watts, that would imply that removing that forcing would cause
a global warming of more than one degree Celsius. That's more than the
warming that we've seen already, so this is a huge factor.

NARRATOR: If we continue as we are, combining reduced air pollution with
an increase in greenhouse gases, temperatures could rise by a further
two or even three degrees Celsius. That's as much as five degrees
Fahrenheit by mid-century, much sooner than current models predict.

JAMES HANSEN: But, in my opinion, three degrees Celsius is not the level
of dangerous interference; that's the level which guarantees disaster.

NARRATOR: James Hansen is particularly worried about what this rise in
temperature would do to the Greenland ice sheet. Even at today's
temperature, there are signs that substantial melting is already
underway.

JAMES HANSEN: It has been overlooked how sensitive ice sheets are to
global temperature. We can see that in the last year, the mass of
Greenland decreased by 200 cubic kilometers of ice. That's a lot of ice.

I cannot imagine that the ice sheets could survive more than a few
centuries with a three-degree Celsius warming. So that would mean a sea
level rise of several meters per century, and it would just continue.
And once that starts, it's out of our control.

NARRATOR: The last time the Earth was three degrees warmer was 3,000,000
years ago, when there was a natural increase in the level of greenhouse
gases in the atmosphere. The geological evidence indicates that melting
ice raised sea levels 25 meters higher than today.

JAMES HANSEN: In the United States, New York City...with a 25-meter sea
level rise, most of the city would be under water; Washington,
D.C....much of it would be under, but there are other regions that would
be...suffer much more; Florida...almost the entire state would be under
water; and, likewise, Louisiana. So we really can't afford to go down
that path.

NARRATOR: And it would not just be coastlines that would be transformed
if the Earth warms by three degrees. Climate models suggest the Amazon
basin would become much drier and vulnerable to fire. What's left of the
world's greatest tropical rainforest could simply burn away, and, in the
process, release still more carbon dioxide, further accelerating global
warming.

Most models do not yet take full account of the impact of global dimming
and predict warming between two and five degrees Celsius, by the end of
the century. But just as global dimming may have lulled the public and
politicians into a false sense of security about climate change, has it
misled climate scientists about the real power of the greenhouse effect
to change global temperatures?

Peter Cox, a leading climate modeler, has come up with a controversial
new analysis based on the observed warming of the last century. If
scientists have underestimated the cooling effects of global dimming in
the past, he believes, they may also be underestimating the heating
effects of global warming in the future.

PETER COX: We've got two competing effects, really, that...we've got the
greenhouse effect, which has tended to warm up the climate, but then
we've got this other effect, that's much stronger than we thought, which
is a cooling effect that comes from particles in the atmosphere. And
they're competing with one another.

And we know the climate's moved to a warmer state by about six-tenths of
a degree over the last hundred years. So the whole thing's moved this
way. If it turns out that the cooling is stronger than we thought, then
the warming, also, is a lot stronger than we thought. And that means the
climate's more sensitive to carbon dioxide than we originally thought,
and it means our models may be under-sensitive to carbon dioxide.

NARRATOR: While today's models foresee a maximum warming of five degrees
Celsius by the end of the century, Cox thinks that it is not beyond the
realms of possibility that by 2100, temperatures could rise by as much
as 10 degrees Celsius, 18 degrees Fahrenheit.

Many plant species could not survive such rapid climate change. In his
scenario, trees would die all over the planet; the world's best
agricultural land would be struck by drought and soil erosion; famine
would not be far behind. And in the far north, there would be a risk of
releasing a vast natural store of greenhouse gas bigger than all the oil
and coal reserves of the planet.

PETER COX: We will be in danger of destabilizing these things called
"methane hydrates," which store a lot of methane at the bottom of the
ocean, in a kind of frozen form-ten thousand billion tons of this
stuff-and they're known to be destabilized by warming.

NARRATOR: If this were to happen, some or all of the ten thousand
billion tons of methane, a greenhouse gas eight times stronger than
carbon dioxide, would be released into the atmosphere. When this last
happened 50,000,000 years ago, when the Earth was already warmer than it
is today, the average temperature rocketed by 13 degrees Fahrenheit,
making the Earth 25 degrees hotter than today, and life struggled to
survive.

Some scientists consider this model extreme, but all climate models
contain important unknowns and ranges of possibility. Our new
understanding of global dimming has complicated the task of forecasting
the future but has also brought the probability of dangerous climate
change much closer.

Today, there's a strong scientific consensus that without urgent action
to reduce our burning of coal, oil and gas, we risk creating a world
very different from the one which has been so hospitable to humanity.