Gary did not forget the early objective and
years later produced a model[7] viable for climate from snowball Earth
to a prelude of the Venus syndrome (Fig. 10.2). This model helps clarify
the “cold trap” effect that limits hydrogen escape. The temperature
profile in Earth’s atmosphere today is near that in 1950 (black curve,
Fig. 10.2), ~15°C at the surface and colder than –60°C at the tropopause
(temperature minimum near the 100 mb[V] pressure level). Extreme cold
at the tropopause “wrings out” almost all water in upward moving air.
Thus, little water makes it to the upper atmosphere above the 1 mb
level, where it can be dissociated by extreme ultraviolet radiation. As a
result, negligible water is escaping from Earth today.
The other curves in Fig. 10.2 are computed for successive doublings of atmospheric CO2. Each CO2
doubling is a “forcing” equivalent to a 2% increase of solar
irradiance, as discussed in later chapters. Our Sun is an ordinary star,
“burning” hydrogen via nuclear fusion, with solar irradiance increasing
10% per billion years, equivalent to 5 CO2 doublings (32×CO2) in 1 billion years. The 20% increase of solar irradiance in two billion years produces a climate forcing equivalent to 1024×CO2.
By then, upwelling circulation in Earth’s atmosphere has a clear path
to pump water directly into the upper atmosphere (Fig. 10.2). As the
Sun’s irradiance continues to grow, Earth will lose its ocean, probably
within 3 billion years, and our own Venus syndrome will commence. There
is no need to be concerned about that. Three billion years is 100
million human generations in the future. If humanity still exists, it
likely will have technology to move a livable environment to a safer
distance from the Sun.
Runaway climate change can occur, if climate feedbacks are large enough. Global warming caused by 2×CO2
or a 2% increase of solar irradiance would be only 1.2°C, if there were
no climate feedbacks, because 1.2°C warming increases radiation to
space enough to restore planetary energy balance. However, observations
and modeling reveal three main feedbacks – increased atmospheric water
vapor, decreased cloud albedo (reflectivity), and decreased sea ice
albedo – and all are amplifying. The net feedback effect is described by
a simple equation,[VI] ΔT = 1.2°C/(1 – g), where ΔT is climate
sensitivity for 2×CO2 and g is the feedback “gain” in our
feedback parlance.[9] Climate system gain today (sum of all feedbacks)
is likely between g = 0.7 (thus ΔT = 4°C) and g = 0.75 (ΔT = 4.8°C), as
we will show in later chapters.
Runaway climate occurs if g approaches unity. Runaway happened several
times when the Sun was weaker, Earth was cooler, and sea ice was
extensive. Atmospheric CO2 varies due to the level of
volcanic emissions and other processes associated with movement of
continental plates. When decline of CO2 caused enough cooling
for sea ice to expand toward the tropics, g reached unity and runaway
to snowball Earth occurred.[10] Eventually, volcanoes increased
atmospheric CO2 enough for sea ice near the equator to melt,
and runaway global warming ensued. Warming then was likely rapid until
the “fuel” for the sea ice feedback (sea ice area) was small enough for
total gain, g, to subside to less than unity. Since the most recent
snowball event, 600 million years ago, the Sun’s irradiance has
increased 6%, making another snowball Earth implausible.
Earth’s paleoclimate history
contains remarkable data on climate change[11] that is still being
converted into knowledge. “Hyperthermal” events, rapid global warming of
a few degrees Celsius,[12] are helpful for understanding the potential
for limited runaway warming. The larger episodic hyperthermal events are
separated by at least a million years; they coincide with, and are
likely triggered by, extreme eccentricity[VII] of Earth’s
orbit.[13] These rapid warmings are marked by changes of the carbon
isotopes in ocean sediments that imply release of hundreds or thousands
of gigatons of isotopically depleted carbon.[VIII] Most interpretations
are that extreme summer heat and drought due to the eccentric orbit lead
to oxidation of the carbon in peat, permafrost, and/or methane
hydrates. Rapid warming caused by the increased atmospheric CO2 then speeds depletion of carbon reservoirs.
In Storms of My Grandchildren,
I painted a scenario in which all fossil fuels are burned rapidly –
within the next 1-2 centuries -- including unconventional ones
(hydrofracturing to extract gas and oil, tar sands, heavy oil). Total
fossil fuel resources are huge, far exceeding proven reserves, which
expand as technology improves. That extreme scenario yields a forcing,
including other greenhouse gases, of 8×CO2. Visitors to Earth
in 2525 found a devastated planet. Is that possible? Land temperature
rises about 1.5 times more than global average, and this scenario could
bring into play feedbacks such as melting permafrost and/or methane
hydrates.
Thus, the conclusion that burning all fossil fuels rapidly would lead to
extreme climate change and pose an existential threat to humanity may
be right, but the discussion in Storms was flawed. First, I did
not distinguish between and explain well the Venus Syndrome and runaway
climate. Second, I inferred runaway warming based on simulations with
our GCM that found an uptick in climate sensitivity between 4×CO2 and 8×CO2 and GCM breakdown for 16×CO2.
The model breakdown, however, was only an indication that one or more
of the scores of processes in the complex GCM was pushed outside its
range of validity. Once we had the model version developed by Gary
Russell – stripped of all unessential processes so that it could be used
to investigate climate sensitivity – the effect of limited “ammunition”
in most feedbacks was clear. Only water vapor has a practically
unlimited source (the ocean). The physics and radiative properties of
water vapor are understood, calculated well in the model, and do not
yield runaway.
The underlying problem soon became clear. My research in the 20 years before writing Storms
was focused on GCMs. In 1989, NASA received funding from Congress for
“Mission to Planet Earth,” an effort to understand ongoing global
change. Our group submitted two proposals: (1) a comprehensive GCM study
of the carbon, energy, and water cycles, and (2) a satellite instrument
to measure aerosol climate forcing. Remarkably, both proposals were
selected.[IX] At the meeting announcing winning proposals, in a gentle
mocking of the ambitious objectives of our GCM investigation, NASA
summarized its title as “The Theory of Everything.”
That summary epitomizes a problem with GCMs. Global modeling is
essential to investigate the simultaneous interactions of all parts of
the global system. However, GCMs are imperfect – at best approximating
the laws of nature – so there are continual efforts to improve the
models and include more physical processes. It is easy to spend most of
one’s time on modeling, crowding out alternative ways to investigate a
problem. It was my own fault, a self-inflicted error, which I did not
recognize until I began to question conclusions of the Intergovernmental
Panel on Climate Change (IPCC), specifically IPCC’s downplaying of the
threat of sea level rise and shutdown of the overturning ocean
circulation.
How could those conclusions be disputed? We needed to go back to a broad
research approach, one that placed comparable emphasis on (1) Earth’s
climate history, (2) global climate modeling, and (3) modern
observations of ongoing climate change, as will be described in later
chapters.
The “runaway” climate threat
now is the danger that today’s accelerated global warming will push
Earth past a “point of no return,” with irreversible consequences for
today’s young people and their descendants. I described the danger of
rapid ice sheet collapse and sea level rise as the “tipping point” in a
December 2005 tribute to Charles David Keeling[14] and Bill McKibben
popularized this a month later in an article[15] in the New York Review. However, Lenton et al.[16] now
use “tipping point” for a broad range of climate feedbacks, many of
which are reversible when the climate forcing is removed or replaced
with global cooling. Therefore, I prefer “point of no return”[1] as
terminology for the point of lock-in of unavoidable ice sheet collapse.
The danger of passing the point of no return is taboo with the United
Nations Intergovernmental Panel on Climate Change (IPCC), the
organization that we should expect to be most protective of the future
of young people. This reticence of IPCC is a cause for concern, which
deserves to be pointed out and vigorously debated. IPCC relies on models
with millennial response times, even when driven by forcings that dwarf
any experienced in Earth’s history. Based on paleoclimate data, global
modeling, and ongoing ocean and ice sheet observations, we have
concluded that shutdown of the ocean’s overturning circulation could
occur within decades and this will affect ocean/ice sheet interactions
and the rate of sea level rise.[17] We will show in later chapters that
up-to-date data support these conclusions. Concern about the danger of
passing the point of no return is not a reason to panic. The climate
system’s delayed response provides time to take preventive actions, if
the science is understood well enough to define effective policy
actions.
Public support will be needed to achieve timely, effective, climate and
energy policy, but, as of now, long-term climate change is far down the
list of public concerns. However, priorities can change – and have
historically[18] – as effects of changing climate on weather increase.
What a coincidence! That brings us back to our chronological account, as
we had a remarkable opportunity to witness the most exciting
development ever in weather prediction.
|
|
No comments:
Post a Comment