Why I must speak out about climate change

Top climate scientist James Hansen tells the story of his involvement in the science of and debate over global climate change. In doing so he outlines the overwhelming evidence that change is happening and why that makes him deeply worried about the future.

What do I know

that would cause me,

a reticent, Midwestern scientist,

to get myself arrested

in front of the White House protesting?

And what would you do

if you knew what I know?

Let's start with how I got to this point.

I was lucky to grow up

at a time when it was not difficult

for the child of a tenant farmer

to make his way to the state university.

And I was really lucky

to go to the University of Iowa

where I could study under Professor James Van Allen

who built instruments

for the first U.S. satellites.

Professor Van Allen told me

about observations of Venus,

that there was intense microwave radiation.

Did it mean that Venus had an ionosphere?

Or was Venus extremely hot?

The right answer,

confirmed by the Soviet Venera spacecraft,

was that Venus was very hot --

900 degrees Fahrenheit.

And it was kept hot

by a thick carbon dioxide atmosphere.

I was fortunate to join NASA

and successfully propose

an experiment to fly to Venus.

Our instrument took this image

of the veil of Venus,

which turned out to be

a smog of sulfuric acid.

But while our instrument was being built,

I became involved in calculations

of the greenhouse effect

here on Earth,

because we realized

that our atmospheric composition was changing.

Eventually, I resigned

as principal investigator

on our Venus experiment

because a planet changing before our eyes

is more interesting and important.

Its changes will affect all of humanity.

The greenhouse effect had been well understood

for more than a century.

British physicist John Tyndall,

in the 1850's,

made laboratory measurements

of the infrared radiation,

which is heat.

And he showed that gasses such as CO2 absorb heat,

thus acting like a blanket

warming Earth's surface.

I worked with other scientists

to analyze Earth climate observations.

In 1981,

we published an article in Science magazine

concluding that observed warming

of 0.4 degrees Celsius

in the prior century

was consistent with the greenhouse effect

of increasing CO2.

That Earth would likely warm in the 1980's,

and warming would exceed

the noise level of random weather

by the end of the century.

We also said that the 21st century

would see shifting climate zones,

creation of drought-prone regions

in North America and Asia,

erosion of ice sheets, rising sea levels

and opening of the fabled Northwest Passage.

All of these impacts

have since either happened

or are now well under way.

That paper was reported on the front page of the New York Times

and led to me testifying to Congress

in the 1980's,

testimony in which I emphasized

that global warming increases both extremes

of the Earth's water cycle.

Heatwaves and droughts on one hand,

directly from the warming,

but also, because a warmer atmosphere

holds more water vapor

with its latent energy,

rainfall will become

in more extreme events.

There will be stronger storms and greater flooding.

Global warming hoopla

became time-consuming

and distracted me from doing science --

partly because I had complained

that the White House altered my testimony.

So I decided to go back

to strictly doing science

and leave the communication to others.

By 15 years later,

evidence of global warming was much stronger.

Most of the things mentioned in our 1981 paper

were facts.

I had the privilege to speak twice

to the president's climate task force.

But energy policies continued to focus

on finding more fossil fuels.

By then we had two grandchildren,

Sophie and Connor.

I decided

that I did not want them in the future

to say, "Opa understood what was happening,

but he didn't make it clear."

So I decided to give a public talk

criticizing the lack of an appropriate energy policy.

I gave the talk at the University of Iowa in 2004

and at the 2005 meeting

of the American Geophysical Union.

This led to calls

from the White House to NASA headquarters

and I was told that I could not give any talks or speak with the media

without prior explicit approval

by NASA headquarters.

After I informed the New York Times

about these restrictions,

NASA was forced to end the censorship.

But there were consequences.

I had been using the first line

of the NASA mission statement,

"To understand and protect the home planet,"

to justify my talks.

Soon the first line of the mission statement

was deleted, never to appear again.

Over the next few years

I was drawn more and more

into trying to communicate the urgency

of a change in energy policies,

while still researching the physics of climate change.

Let me describe the most important conclusion from the physics --

first, from Earth's energy balance

and, second, from Earth's climate history.

Adding CO2 to the air

is like throwing another blanket on the bed.

It reduces Earth's heat radiation to space,

so there's a temporary energy imbalance.

More energy is coming in

than going out,

until Earth warms up enough

to again radiate to space

as much energy as it absorbs from the Sun.

So the key quantity

is Earth's energy imbalance.

Is there more energy coming in

than going out?

If so, more warming is in the pipeline.

It will occur without adding any more greenhouse gasses.

Now finally,

we can measure Earth's energy imbalance precisely

by measuring the heat content

in Earth's heat reservoirs.

The biggest reservoir, the ocean, was the least well measured,

until more than 3,000 Argo floats

were distributed around the world's ocean.

These floats reveal

that the upper half of the ocean

is gaining heat at a substantial rate.

The deep ocean is also gaining heat at a smaller rate,

and energy is going

into the net melting of ice all around the planet.

And the land, to depths of tens of meters,

is also warming.

The total energy imbalance now

is about six-tenths of a watt per square meter.

That may not sound like much,

but when added up over the whole world, it's enormous.

It's about 20 times greater

than the rate of energy use by all of humanity.

It's equivalent to exploding

400,000 Hiroshima atomic bombs per day

365 days per year.

That's how much extra energy

Earth is gaining each day.

This imbalance,

if we want to stabilize climate,

means that we must reduce CO2

from 391 ppm, parts per million,

back to 350 ppm.

That is the change needed to restore energy balance

and prevent further warming.

Climate change deniers argue

that the Sun is the main cause of climate change.

But the measured energy imbalance occurred

during the deepest solar minimum in the record,

when the Sun's energy reaching Earth was least.

Yet, there was more energy coming in than going out.

This shows that the effect of the Sun's variations on climate

is overwhelmed by the increasing greenhouse gasses,

mainly from burning fossil fuels.

Now consider Earth's climate history.

These curves for global temperature,

atmospheric CO2 and sea level

were derived from ocean cores and Antarctic ice cores,

from ocean sediments and snowflakes

that piled up year after year

over 800,000 years

forming a two-mile thick ice sheet.

As you see, there's a high correlation

between temperature, CO2 and sea level.

Careful examination shows

that the temperature changes

slightly lead the CO2 changes

by a few centuries.

Climate change deniers like to use this fact

to confuse and trick the public

by saying, "Look, the temperature causes CO2 to change,

not vice versa."

But that lag

is exactly what is expected.

Small changes in Earth's orbit

that occur over tens to hundreds of thousands of years

alter the distribution

of sunlight on Earth.

When there is more sunlight

at high latitudes in summer, ice sheets melt.

Shrinking ice sheets

make the planet darker,

so it absorbs more sunlight

and becomes warmer.

A warmer ocean releases CO2,

just as a warm Coca-Cola does.

And more CO2 causes more warming.

So CO2, methane, and ice sheets

were feedbacks

that amplified global temperature change

causing these ancient climate oscillations to be huge,

even though the climate change was initiated

by a very weak forcing.

The important point

is that these same amplifying feedbacks

will occur today.

The physics does not change.

As Earth warms,

now because of extra CO2 we put in the atmosphere,

ice will melt,

and CO2 and methane will be released

by warming ocean and melting permafrost.

While we can't say exactly how fast

these amplifying feedbacks will occur,

it is certain they will occur,

unless we stop the warming.

There is evidence

that feedbacks are already beginning.

Precise measurements

by GRACE, the gravity satellite,

reveal that both Greenland and Antarctica

are now losing mass,

several hundred cubic kilometers per year.

And the rate has accelerated

since the measurements began

nine years ago.

Methane is also beginning

to escape from the permafrost.

What sea level rise

can we look forward to?

The last time CO2 was 390 ppm,

today's value,

sea level was higher

by at least 15 meters, 50 feet.

Where you are sitting now

would be under water.

Most estimates are that, this century,

we will get at least one meter.

I think it will be more

if we keep burning fossil fuels,

perhaps even five meters, which is 18 feet,

this century or shortly thereafter.

The important point

is that we will have started a process

that is out of humanity's control.

Ice sheets would continue to disintegrate for centuries.

There would be no stable shoreline.

The economic consequences are almost unthinkable.

Hundreds of New Orleans-like devastations

around the world.

What may be more reprehensible,

if climate denial continues,

is extermination of species.

The monarch butterfly

could be one of the 20 to 50 percent of all species

that the Intergovernmental Panel on Climate Change estimates

will be ticketed for extinction

by the end of the century

if we stay on business-as-usual fossil fuel use.

Global warming is already affecting people.

The Texas, Oklahoma, Mexico

heatwave and drought last year,

Moscow the year before

and Europe in 2003,

were all exceptional events,

more than three standard deviations outside the norm.

Fifty years ago,

such anomalies

covered only two- to three-tenths

of one percent of the land area.

In recent years,

because of global warming,

they now cover about 10 percent --

an increase by a factor of 25 to 50.

So we can say with a high degree of confidence

that the severe Texas and Moscow heatwaves

were not natural;

they were caused by global warming.

An important impact,

if global warming continues,

will be on the breadbasket of our nation and the world,

the Midwest and Great Plains,

which are expected to become prone to extreme droughts,

worse than the Dust Bowl,

within just a few decades,

if we let global warming continue.

How did I get dragged deeper and deeper

into an attempt to communicate,

giving talks in 10 countries, getting arrested,

burning up the vacation time

that I had accumulated over 30 years?

More grandchildren helped me along.

Jake is a super-positive,

enthusiastic boy.

Here at age two and a half years,

he thinks he can protect

his two and a half-day-old little sister.

It would be immoral

to leave these young people

with a climate system

spiraling out of control.

Now the tragedy about climate change

is that we can solve it

with a simple, honest approach

of a gradually rising carbon fee

collected from fossil fuel companies

and distributed 100 percent electronically

every month to all legal residents

on a per capita basis,

with the government not keeping one dime.

Most people would get more in the monthly dividend

than they'd pay in increased prices.

This fee and dividend

would stimulate the economy

and innovations,

creating millions of jobs.

It is the principal requirement

for moving us rapidly

to a clean energy future.

Several top economists

are coauthors on this proposition.

Jim DiPeso of Republicans for Environmental Protection

describes it thusly:

"Transparent. Market-based.

Does not enlarge government.

Leaves energy decisions to individual choices.

Sounds like a conservative climate plan."

But instead of placing a rising fee on carbon emissions

to make fossil fuels pay

their true cost to society,

our governments are forcing the public

to subsidize fossil fuels

by 400 to 500 billion dollars

per year worldwide,

thus encouraging extraction of every fossil fuel --

mountaintop removal,

longwall mining, fracking,

tar sands, tar shale,

deep ocean Arctic drilling.

This path, if continued,

guarantees that we will pass tipping points

leading to ice sheet disintegration

that will accelerate out of control of future generations.

A large fraction of species

will be committed to extinction.

And increasing intensity of droughts and floods

will severely impact breadbaskets of the world,

causing massive famines

and economic decline.

Imagine a giant asteroid

on a direct collision course with Earth.

That is the equivalent

of what we face now.

Yet, we dither,

taking no action

to divert the asteroid,

even though the longer we wait,

the more difficult and expensive it becomes.

If we had started in 2005,

it would have required emission reductions of three percent per year

to restore planetary energy balance

and stabilize climate this century.

If we start next year,

it is six percent per year.

If we wait 10 years, it is 15 percent per year --

extremely difficult and expensive,

perhaps impossible.

But we aren't even starting.

So now you know what I know

that is moving me to sound this alarm.

Clearly, I haven't gotten this message across.

The science is clear.

I need your help

to communicate the gravity and the urgency

of this situation

and its solutions

more effectively.

We owe it to our children and grandchildren.

Thank you.

(Applause)