We are deeply disturbed
by the recent escalation of political assaults on scientists in
general and on climate scientists in particular. All citizens
should understand some basic scientific facts. There is always some
uncertainty associated with scientific conclusions; science never
absolutely proves anything. When someone says that society should
wait until scientists are absolutely certain before taking any
action, it is the same as saying society should never take action.
For a problem as potentially catastrophic as climate
change, taking no action poses a dangerous risk for our
planet.

Scientific conclusions derive from an understanding of basic
laws supported by laboratory experiments, observations of nature,
and mathematical and computer modelling. Like all human beings,
scientists make mistakes, but the scientific process is designed to
find and correct them. This process is inherently adversarial-
scientists build reputations and gain recognition not only for
supporting conventional wisdom, but even more so for demonstrating
that the scientific consensus is wrong and that there is a better
explanation. That’s what Galileo, Pasteur, Darwin, and Einstein
did. But when some conclusions have been thoroughly and deeply
tested, questioned, and examined, they gain the status of
“well-established theories” and are often spoken of as “facts.”

For instance, there is compelling scientific evidence that our
planet is about 4.5bn years old (the theory of the origin of
Earth), that our universe was born from a single event about 14bn
years ago (the Big Bang theory), and that today’s organisms evolved
from ones living in the past (the theory of evolution). Even as
these are overwhelmingly accepted by the scientific community, fame
still awaits anyone who could show these theories to be wrong.
Climate change now falls into this category: there is compelling,
comprehensive, and consistent objective evidence that humans are
changing the climate in ways that threaten our societies and the
ecosystems on which we depend.

Many recent assaults on climate science and, more
disturbingly, on climate scientists by climate change deniers, are
typically driven by special interests or dogma, not by an honest
effort to provide an alternative theory that credibly satisfies the
evidence.

Many recent assaults on climate science and, more disturbingly,
on climate scientists by climate change deniers, are typically
driven by special interests or dogma, not by an honest effort to
provide an alternative theory that credibly satisfies the evidence.
The Intergovernmental Panel on Climate Change (IPCC) and other
scientific assessments of climate change, which involve thousands
of scientists producing massive and comprehensive reports, have,
quite expectedly and normally, made some mistakes. When errors are
pointed out, they are corrected.

But there is nothing remotely identified in the recent events
that changes the fundamental conclusions about climate change:

(i) The planet is warming due to
increased concentrations of heat-trapping gases in our atmosphere.
A snowy winter in Washington does not alter this fact.

(ii) Most of the increase in the
concentration of these gases over the last century is due to human
activities, especially the burning of fossil fuels and
deforestation.

(iii) Natural causes always play a
role in changing Earth’s climate, but are now being overwhelmed by
human-induced changes.

(iv) Warming the planet will cause
many other climatic patterns to change at speeds unprecedented in
modern times, including increasing rates of sea-level rise and
alterations in the hydrologic cycle. Rising concentrations of
carbon dioxide are making the oceans more acidic.

(v) The combination of these complex
climate changes threatens coastal communities and cities, our food
and water supplies, marine and freshwater ecosystems, forests, high
mountain environments, and far more.

Much more can be, and has been, said by the world’s scientific
societies, national academies, and individuals, but these
conclusions should be enough to indicate why scientists are
concerned about what future generations will face from business-
as-usual practices. We urge our policymakers and the public to move
forward immediately to address the causes of climate change,
including the unrestrained burning of fossil fuels.

We also call for an end to McCarthy- like threats of criminal
prosecution against our colleagues based on innuendo and guilt by
association, the harassment of scientists by politicians seeking
distractions to avoid taking action, and the outright lies being
spread about them. Society has two choices: we can ignore the
science and hide our heads in the sand and hope we are lucky, or we
can act in the public interest to reduce the threat of global
climate change quickly and substantively. The good news is that
smart and effective actions are possible. But delay must not be an
option.

• The signatories are all members of the US National Academy
of Sciences but are not speaking on its behalf or on behalf of
their institutions.