Mars, Venus and Earth
are the Goldilocks planets – too cold, too hot, and just right. These
planets reveal how a planet’s surface temperature depends on atmospheric
gases as well as the planet’s distance from the Sun. The physics is
energy balance: a planet sends back to space, as (infrared) heat
radiation, the energy in sunlight that it absorbs. Absorbed solar energy
depends simply on the Sun’s irradiance[I] the planet’s distance from
the Sun, and the fraction of incident sunlight that the planet absorbs
(the remainder being reflected). The surface temperature is then given
by the Stefan-Boltzmann law[1] (physical principle), if the planet has
no atmosphere.
If the planet has an atmosphere that partly blocks heat emission, the
surface must be warmer than given by the Stefan-Boltzmann equation for
emission to space to match absorbed solar energy.[2] This “greenhouse”
warming depends on how opaque the atmosphere is in the infrared. Mars’
atmosphere has small infrared opacity, so its surface is only a few
degrees warmer than it would be with no atmosphere. Venus has a thick
atmosphere of (96% CO2) with sulfuric acid clouds and water vapor that absorb at wavelengths where CO2
absorption is weak; resulting greenhouse warming on Venus is about
500°C. Earth has intermediate greenhouse warming, about 33°C, enough to
change Earth from an ice ball at –19°C (–2°F) to a hospitable +14°C
(57°F).
How did these planets get to this situation? Can Earth end up like
Venus, a lifeless hothouse? Yes, it can, but the runaway greenhouse
story described in Storms of My Grandchildren needs correction. To explain the change, we should first discuss what happened on Venus.
Venus reached an extreme,
“baked crust,” greenhouse state. Venus was doomed to permanent climatic
hell once it lost its ocean. On a planet with an ocean, CO2
injected into the air by volcanoes (or humans) gets put back into the
crust[II] within millennia, a geologically rapid time scale. Removal
from the air occurs due to weathering: rainfall is slightly acidic (from
dissolved atmospheric CO2), and plants and animals release
acidic compounds, which speeds weathering of rocks. Streams and rivers
carry chemicals to the ocean, where CO2 is deposited on the ocean floor as limestone. Ocean-free Venus, in contrast, has no mechanism for rapid return of volcanic CO2 to the crust. So much CO2
is now baked out of the Venus crust and mantle that the surface
pressure is 90 times that on Earth, enough to crush any human visitors,
if they were not already fried to a cinder.
Why the big difference between these neighboring planets? The Sun, and
the planets orbiting it, formed 4.6 billion years ago from gravitational
collapse of a swirl of gas, ice and dust in a spiral arm of our Galaxy,
the Milky Way. Thus, Venus and Earth are made of the same material. As
the rotating mass of the proto-Sun grew, collapsed, and “ignited” with
nuclear fusion of hydrogen, a hot solar wind of electrically charged
atoms blew away the original atmospheres of the four inner planets
because of their proximity to the Sun and their weak gravity. The
atmospheres of Venus and Earth today are secondary[III] gases released
on geologic time scales from the mantle and crust. Thus, it is likely
that Venus once had an ocean, even though Venus is very dry today.[3]
The key to resolving the mystery – why Earth retains an ocean, but Venus
does not – lies in escape of hydrogen from the Venus upper atmosphere.
That conclusion is confirmed by the great over-abundance of deuterium
(heavy hydrogen) in the Venus atmosphere relative to protium, the main
hydrogen isotope; it is more difficult for heavier isotopes to escape a
planet’s gravity,[4] even though escape occurs via several
mechanisms.[5],[6] Rapid hydrogen escape cannot occur on Earth today
because our upper atmosphere contains almost no water. Temperature at
the tropopause, the boundary between the troposphere and stratosphere at
height ~10 miles, is about −60°C. Such low temperature “wrings out”
almost all the water from upward moving air.
Can Earth’s water escape in the future if Earth becomes hotter? Andy
Ingersoll[7] described a potential “runaway” greenhouse effect that is
relevant to Earth, as well as Venus. Ingersoll’s paper, in 1969,
introduced the basic issue: can a critical point be reached at which
water loss from a planet becomes self-sustaining and rapid?
Investigation of planetary evolution provides a perspective that
increases understanding of humanmade climate change.
The planetary perspective
provides insights about climate change far beyond those of the simple
Goldilocks planetary comparison. The Earth science community has long
had a broad perspective in climate analysis, with a research approach
that emphasizes the value of studying climate change via the combined
use of paleoclimate data, modern observations, and global climate
modeling. Planetary evolution provides a fourth way to assess climate
sensitivity and climate change, while providing insights about each of
these other three approaches. This planetary perspective also helps
identify physical processes that are most important in determining the
magnitude of climate sensitivity. Remarkably, the planetary perspective
also helps to clarify the causes of increasing regional climate extremes
today.
Those are extraordinary assertions! It is tempting to describe the
planetary topic further in this chapter, but that would require a
writer’s double sin: a flash-forward in time and use of scientific
graphs that deter some readers. A more effective approach is to keep the
narrative chronological and introduce climate concepts at a pace that
allows more people to appreciate the scientific story. That is the
approach we will take to clarify the planetary perspective.
Will Earth ever have a Venus-like runaway greenhouse? Not on a time
scale people care about. The qualitative reason – a damping effect of
Earth’s massive ocean – was described above. Quantitative evaluation and
implications of the planetary perspective are included in later
chapters. Runaway cannot occur until after the Sun is bright
enough[IV] to drive the loss of Earth’s ocean, and the very soonest that
will occur is several hundred million years in the future. The Venus
syndrome is not the issue for today’s young people.
The “runaway climate” threat is
the danger of passing a “point of no return,” with enormous,
irreversible, consequences for today’s young people and their
descendants. I depicted the danger of rapid ice sheet collapse and sea
level rise as a “tipping point” twenty years ago (Hansen),[8] but
also as a “point of no return.”[9] The latter terminology is now more
appropriate, as Lenton et al.[10] use “tipping point” for a broader
range of amplifying climate feedbacks, many of which are reversible, if
the forcings driving global warming are removed or replaced with global
cooling.
The runaway climate threat and danger of passing a point of no return
are taboo with the United Nations Intergovernmental Panel on Climate
Change (IPCC), the organization that we should expect to be most
protective of the rights and the future of young people. This reticence
of IPCC is a cause for concern, which deserves to be pointed out and
vigorously debated. We have presented evidence that the millennial time
scale of ice sheet changes in the models that IPCC relies on are much
slower than indicated by real-world data, even when ocean and ice sheet
changes are driven by very slowly changing paleoclimate
forcings.[11] Specifically, paleoclimate data, global modeling, and
ongoing ocean and ice sheet observations raise concern that rapid
shutdown of the ocean’s overturning circulation could occur within
decades, which can affect ocean/ice sheet interactions and the rate of
sea level rise.
The potential of passing the point of no return is cause for concern,
but no reason to panic. The climate system’s delayed response to
human-made drivers of climate change – which gives rise to the runaway
climate threat – also provides time to take preventive actions, if the
climate science is understood well enough to define realistic,
effective, policy actions. It is incumbent on us to help define the
research that is needed to better assess the threat of shutdown of ocean
overturning circulation and large sea level rise because of their
irreversible nature.
There is a more mundane, slowly growing climate threat, which the public
is beginning to recognize. Extremes of the hydrologic cycle are
amplified on a hotter planet. Where rainfall occurs, it includes more
extreme rainfall, floods, and stronger storms driven by a warmer ocean,
greater latent energy in water vapor, and larger temperature gradients.
High temperature causes dry times and places to have more extreme heat
waves, droughts and fires. Some tropical regions – and the subtropics in
summer – can become so warm that the human body is unable to cool
itself and survive in the outdoors. If we allow global climate to go
down that path, pressures for emigration from low latitudes would dwarf
the emigration pressures of today, which would likely make the planet
ungovernable.
The scientific method
of investigation must be our guide as we seek a route to a healthy
climate, but what is the scientific method? An objective search for the
truth, for sure, but how do we go about that? The scientific method is
not taught by rote in a class. It is learned more by osmosis from good
scientists. My suggested guidelines are (1) study all available data on a
topic, (2) be skeptical of your interpretation and reassess as new data
become available, and (3) do not let your ideology, your preferences,
affect your assessment. This last point is easier if one is a political
independent. Let’s get on with the story chronology and continually try
to be objective.
What was the error in Storms of My Grandchildren? It was a
statement that extraterrestrial visitors to Earth in 2525 witnessed
boiling tropical oceans. That is not possible on a timescale less than
several hundred million years, as shown later in this book. The error
was caused by overreliance on global climate models and
misinterpretation of a sharp upturn of climate sensitivity in such a
model. Climate models are an essential tool in climate analysis, but we
can obtain more definitive conclusions with comparable efforts in
paleoclimate studies, modern observations, and even the planetary
perspective.
Speaking of science – space science, in particular – my interest in
becoming a scientist-astronaut began to wane in the early 1970s. I
communicated with Brian O’Leary, who had successfully applied to the
first astronaut selection process in 1967 – precisely the class for
which the application deadline coincided with my doctoral thesis
defense. Brian resigned from the astronaut program one year after being
accepted into it, his main reason being realization that it is not
possible to be an astronaut and at the same time be a fulltime,
practicing, scientist. Also, I felt that I still needed to catch up in
planetary science – Jim Pollack had a broader understanding than I did. I
thought that I could catch up, and that Anniek and I could start living
like normal people, and she believed me. I did not foresee the
emergence of more riveting issues, which began with a seemingly
innocuous involvement in weather prediction.
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