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the climate issue and of the Paris Agreement itself are first briefly


The Climate Science Context

and the Paris Agreement

Despite occasional claims to the contrary, the physical science

underlying concerns about climate change is well established.

Carbon dioxide traps heat (i.e., energy) in the atmosphere, a

physical property readily measured through spectroscopic

analysis. Fossil fuels and organic materials such as wood release

carbon dioxide when combusted, increasing atmospheric carbon

dioxide levels and trapping more energy in the atmosphere.

This results not only in higher global average surface tempera-

tures; it also, in effect, puts weather on steroids, with impacts

that include more-intense droughts and deluges. In addition, as

higher temperatures trigger

thermal expansion of water

and melting of land-based

glaciers and ice sheets,

average sea levels rise.

How big will these

impacts be and how fast

will they come? Will they be

modest and slow enough

that most societies can

adapt (despite dispro-

portionate consequences

for some locales and the

poor)? Or will big changes

come swiftly, with wrenching consequences that destabilize

entire regions and eventually the world economy? The answer

depends primarily on howmuch more carbon pollution is loaded

into the atmosphere. Beyond that, the complexity of the planet’s

climate systemmakes precise answers impossible, at least for

now, so projections of temperature increases and impacts are

best expressed as probability ranges rather than a single point.

The higher but all-too-plausible ends of those ranges paint a pic-

ture of brutal temperature swings, massive droughts and resulting

food insecurity, and rising sea levels that devastate coastal cities

around the globe.

Nor is it plausible to wait and see whether impacts approach

intolerable levels, and only then start reducing emissions. Excess

carbon dioxide persists in the atmosphere for centuries and some

tipping points, such as large-scale melting of polar ice sheets and

carbon-rich Arctic permafrost, would be essentially irreversible,

so impacts would intensify for many years even after emissions

were curtailed. Socioeconomic factors also make turn-on-a-dime

responses infeasible: Reconfiguring the world’s fossil-fueled

infrastructure for energy, transport and industry is the work of

decades, as is upgrading energy efficiency in billions of energy-

consuming buildings and appliances. (While some researchers

are exploring “geoengineering”—artificial manipulation of the

earth’s climate—to rapidly deflect warming, its practicability

remains unclear, to say nothing of its potential unintended con-

sequences.) And the argument that our descendants will be rich

enough to readily adapt to the consequences of a changing cli-

mate ignores the very real possibility that severe climate change

will itself derail future economic growth.

The good news is that the cost of low-carbon energy, particu-

larly renewables, has plummeted over the last decade, spurring

increased deployment. Globally, renewables now account for the

majority of new electric-

power generating capacity.

The bad news? Fossil fuels

today still provide more

than 85 percent of the

world’s total energy supply.

Dramatic emission reduc-

tions require not only that

existing infrastructure be

reconfigured, but also that

new development take a

low-carbon pathway.

Growing recognition of

the urgent need for climate

action prompted enough countries to join the Paris Agreement

by October 2016 to reach its entry-into-force threshold—55 coun-

tries representing 55 percent of global emissions—with near-

unprecedented speed, even as the world was experiencing the

hottest year on record for the third year in a row. (In September

2016, President Barack Obama used a combination of inherent

presidential authority and authority conferred by legislation and

treaty to enter into the Paris Agreement, designating it an execu-

tive agreement not requiring Senate advice and consent.)

Though by nomeans perfect, the Paris Agreement consti-

tutes a major step forward. It articulates a global goal of keeping

temperature increases to “well below” two degrees Celsius above

pre-industrial levels (with efforts to limit warming to 1.5 degrees),

a target initially proposed by scientists as avoiding the worst effects

of climate change.

Contrary to the assertions President Donald Trump made

when announcing the U.S. withdrawal, the Paris Agreement

The Paris Agreement neither

dictates a U.S. emissions

target nor imposes financial

contributions; targets are

nationally determined, and

contributions are voluntarily

decided by each country.