[Based on data since 2000 alone, global warming is still occurring
at a whopping 7-sigma significance. How hot will planet Earth get?]
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THE INCONVENIENT TRUTH OF GLOBAL WARMING IN THE 21ST CENTURY  
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Ethan Siegel
November 10, 2022
Big Think
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_ Based on data since 2000 alone, global warming is still occurring
at a whopping 7-sigma significance. How hot will planet Earth get? _

A hypothetical map of what an ice-free Earth would look like, with
all glaciers, ice sheets and icecaps fully melted. The mean sea level
is a full 67.5 meters (221.5 feet) higher than it is today. Not a
foregone conclusion in coming millennia., Kevin Gill/flickr

 

KEY TAKEAWAYS

* Toward the end of the 20th century, there was a lot of dissent and
argument over the scientifically robust evidence that showed the Earth
is warming. 
* The evidence of this warming is so strong that even if we begin in
the year 2000, it's robust at the 7-sigma level, with less than
1-in-100 billion chance of it being a statistical fluke. 
* We're now at the point that we have to ask ourselves when we want
Earth's hottest year of the 3rd millennium to be? We're capable of
determining the answer.

Back in 1990, with 110 years of temperature records behind them, the
world’s top climate scientists convened to put together a report on
the state of the Earth’s climate. Working collaboratively, the
fruits of their labor became the very first Intergovernmental Panel on
Climate Change (IPCC) report. It definitively showed:

* there had been a global increase in carbon dioxide concentrations
since pre-industrial times from ~280 parts-per-million (ppm) to 354
ppm,
* this was coupled with a global average temperature increase of 0.7
°C (1.3 °F),
* that the increase in temperature was being driven not by the Sun,
volcanoes, or urbanization, but rather by the human-wrought changes to
our atmospheric contents,
* and that this problem would continue to worsen unless carbon
dioxide emissions were curbed.

Despite sounding the alarm
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the past three decades have led to a far more dire situation. As
identified in 2021’s 6th IPCC report
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now sit at 412 ppm, Earth’s average temperature is a full 1.3 °C
(2.3 °F) above pre-industrial levels, and our global carbon emissions
have increased to a new all-time high
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of carbon dioxide annually, up from 22 billion in 1990. The best time
to act was long ago, but the second best time to act is now. Here are
the truths of the matter that everyone who’s vested in following
what the science shows should know.

Yearly surface temperature compared to the 20th-century average from
1880–2021. Blue bars indicate cooler-than-average years; red bars
show warmer-than-average years. The past 7 years on record hold the
top 7 spots for the warmest year on record. Credit: NOAA National
Centers for Environmental information, Climate at a Glance: Global
Time Series

1.) THE EARTH REALLY HAS WARMED SINCE PRE-INDUSTRIAL TIMES, AND THE
RATE OF WARMING IS INCREASING WITH TIME.

The Earth, at present, is indisputably a warmer planet today than at
any point in all of recorded human history. This isn’t because of
the Sun; it isn’t because of Milankovitch cycles
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it isn’t because of volcanic activity. It’s directly due to the
human-caused emission of greenhouse gases, with the concentration of
carbon dioxide being the dominant driving factor in increasing the
Earth’s temperature.

The Sun outputs energy, the Earth orbits at a specific distance from
the Sun, where it absorbs some of that sunlight and reflects the rest,
and then re-radiates its absorbed heat back into space. If we simply
accounted for these factors and ignored Earth’s atmosphere,
including:

* cloud cover,
* greenhouse gases like carbon dioxide, methane, and water vapor,
* and the transparent-in-visible-light but
absorptive-in-infrared-light properties of those gases,

we’d calculate that Earth’s average temperature should be 255
Kelvin (-18 °C / 0 °F). Instead, owing to these effects, Earth’s
average temperature is 288 Kelvin (15 °C / 59 °F). The 33 K
greenhouse effect
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50% attributable to water vapor, 25% due to clouds, 20% due to carbon
dioxide, and 5% to other gases. Increasing the carbon dioxide level by
~50% over its pre-industrialization value is the primary driver of the
recently observed warming.

Using publicly available data from NOAA, one can plot the global
average land-and-ocean temperature anomaly, compared to the 20th
century average, from the year 2000 to the present. The data is so
strongly indicative of warming that a no-warming scenario can be ruled
out with greater than 7-sigma confidence. (Credit: E. Siegel; data,
NOAA National Centers for Environmental information)

2.) IT’S SO SIGNIFICANT THAT IF WE ONLY LOOK AT GLOBAL TEMPERATURES
SINCE 2000, THE SIGNAL IS ROBUST, SIGNIFICANT, AND TERRIFYING.

It took over 100 years from the time when global average temperatures
began being measured back in 1880 before a robust, 5σ signal (with
less than 0.0001% chance of it being a statistical fluke) demonstrated
that the Earth was, in fact, warming. The start of the 21st century
has shown an alarming increase in this rate of warming. In fact, since
the year 2000:

* global average temperatures have increased by 0.47 °C (0.84 °F),
* which is triple the average 20th century warming rate,
* with a “no warming” scenario excluded at a whopping 7σ
significance already (1-in-300 billion odds of being a fluke).

Things are not only getting worse, but the rate at which they’re
getting worse is increasing substantially. We may be the first species
in history to scientifically realize how our activities are impacting,
polluting, and damaging our environment in an existential way. It
remains to be seen whether we’re up to the challenge of addressing
it.

The predictions of various climate models over the years (colored
lines) as compared with the observed global average temperature
(black, thick line). Note how thoroughly well these models performed
historically, and how well the observations continue to fit the data.
(Credit: Z. Hausfather et al., Geophys. Res. Lett, 2019)

3.) THE MAJOR PREDICTIONS OF CLIMATE SCIENCE FROM OVER 50 YEARS AGO
— INCLUDING THE HOCKEY STICK AND WARMING TRENDS — WERE ALL
CORRECT.

The above graph shows a variety of climate models and their
predictions for how the global mean temperature would increase, going
as far back as 1970 and showing the prediction beginning the year they
were made. The black, thick line shows the actual observed temperature
on a year-to-year basis. As you can clearly see with a visual
inspection, or as you can robustly find with a more detailed
statistical analysis, historical climate models and past IPCC
projections have accurately predicted real-world temperatures in
subsequent years.

It’s more than just the warming trends, however. The “hockey
stick” graph now goes back more than 2000 years and shows
definitively how unprecedented the modern warming trend is
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the first two centuries of the 3rd millennium will be warmer than any
multi-century period over the past 100,000 years. Skeptics often
question how much of the warming is due to natural factors versus how
much is due to human activity, and the latest report has answered
that: approximately ~95–100% is human-caused; approximately ~0–5%
is natural (due to solar and volcanic effects). Humans are the cause
of this unprecedented warming, and it is up to us to be the solution,
too.

The past 2020 years of temperature (left) clearly shows the hockey
stick nature of global temperature. The past 170 years (right) shows
the observed temperature variations and the simulated impacts of
natural variability (blue) and human impacts (orange) added to the
natural effects. 95–100% of the warming is due to human activity.
(Credit: IPCC AR6, 2021)

4.) THE KEY TO STOPPING FURTHER, FUTURE WARMING RELIES ON DECREASING
HUMAN-CAUSED CARBON DIOXIDE EMISSIONS.

One of the key things you have to realize is that what we call “the
greenhouse effect” is a little bit of a misnomer; it’s actually
more like a “blanket” effect. The heat-trapping gases in our
atmosphere behave like blankets, keeping the heat our planet emits
here on our planet for longer than it would’ve been kept here in the
absence of those gases. The more gases there are — i.e., the more
“blankets” we continue to put onto our planet — the more we’ll
re-absorb and re-absorb and re-absorb that heat, over and over, before
finally emitting it back into space for good.

All attempts at geoengineering a solution
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simultaneously reducing carbon dioxide emissions, including:

* reflecting sunlight back into space before it’s incident on the
top of Earth’s atmosphere,
* seeding clouds to reflect sunlight back into space,
* injecting aerosols into the stratosphere of Earth,
* sequestering carbon in either natural (i.e., trees) or artificial
(i.e., captured carbon) stores,
* and enhancing the alkalinity of the ocean,

are stopgap measures at best, where the underlying cause of the
problem continues to be exacerbated. “Ceasing to put new blankets
on” has to be part of a sustainable solution to human-caused climate
change.

The five expected warming/emissions scenarios and the corresponding
sea-level rise, as detailed in the 6th IPCC report. If the Antarctic
ice sheet becomes unstable, a possibility in the highest-emission
scenarios, the impacts will be much more severe, as indicated by the
dotted line. (Credit: IPCC AR6, 2021)

5.) SEA LEVELS WILL RISE THROUGHOUT THE 3RD MILLENNIUM BY ANYWHERE
FROM 2 METERS TO UP TO 22 METERS, DEPENDING ON OUR CURRENT AND
NEAR-FUTURE ACTIONS.

The biggest risk, of course, is that an abrupt and colossal change
will occur: what we’ve traditionally called a “tipping point.”
Two of these are intimately related to the seas: ice sheet collapse
and abrupt changes in ocean circulation. Most climate projections
result in 0.5–1.0 meters of sea level rise through 2100, with an
acceleration occurring in the moderate and high-end emissions
scenarios. However, a destabilized Antarctic ice sheet (shown on the
above graph as a dotted line) will lead to an ensuing catastrophic
rise in sea levels. According to the IPCC
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“Global mean sea level rise above the likely
range — approaching 2 m by 2100 and 5 m by 2150 under a very
high… emissions scenario — cannot be ruled out due to deep
uncertainty in ice sheet processes.”

Coastal flooding events will occur more frequently, particularly along
the Gulf and Atlantic coasts of the United States. A combination of
sea level rise, tidal cycles, and shifting ocean currents could
transform what were once-in-a-century flooding events in the 1900s
into annual events by 2100. The report also warns that the combination
of deep ocean warming and ice sheet melt will likely, with high
confidence, result in elevated sea levels for millennia. If warming is
limited to 1.5 °C, 2 °C, or 5 °C over the next few centuries, we
can still expect sea levels to rise by 2–3 m, 2–6 m, or 19–22 m,
respectively, by the year 3000.

A comparison of the projected global temperatures into the future
alongside several projected global carbon emissions scenarios. The
higher and mid-high scenarios are still in play; the “even lower
scenario” is looking unlikely given the present state of affairs.
(Credit: Katharine Hayhoe/NOAA/Climate.gov)

6.) 1.5 °C (2.7 °F) WARMING IS INEVITABLE AND WILL OCCUR BY THE
EARLY 2030S, BUT ISN’T “GAME OVER” FOR THE CLIMATE.

The IPCC, in their most recent report
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scenarios. Each one, individually, was driven by predictions for the
future of carbon emissions on planet Earth.

* SSP1–1.9 was the scenario chosen to limit temperatures in 2100 to
below 1.5 °C warming,
* SSP1–2.6 was the most plausible low-end emissions scenario
considered,
* SSP2–4.5 was a scenario that assumed that policies adopted in
2021-era Europe would be successfully implemented worldwide,
* SSP3–7.0, represented a high-end emissions pathway where
emissions slowly increase, doubling by century’s end,
* and SSP3–8.5 represented the most pessimistic high-end emissions
scenario considered.

In these 5 scenarios, respectively, it’s estimated that the
Earth’s global average temperature in the year 2100 will be 1.4 °C,
1.8 °C, 2.7 °C, 3.6 °C, and 4.4 °C warmer than the 20th century
average.

In all cases, we will pass the 1.5 °C mark sometime in the early
2030s: about 10 years from now. In all but the two most optimistic
scenarios for emissions, we will pass 2.0 °C of warming sometime
around 2050. None of this, however, means “game over” for the
climate. Temperature trends should reverse once emissions go
carbon-negative. The less time we spend with extreme temperatures on
Earth, the more successful we’ll be at mitigating the ill effects of
climate change; the more rapidly we decarbonize the energy sector, the
better those mitigations will be.

Carbon emissions under the 5 scenarios considered by the IPCC in their
6th (2021) report. Emissions are measured in billions of tons of
carbon dioxide per year (y-axis). The blue scenarios are dependent on
immediate and rapid decarbonization, while the orange line assumes
substantial steps occurring in short order to reduce emissions. The
redder scenarios are far more troubling, and possibly a more realistic
reflection of the status quo. (Credit: IPCC AR6, 2021)

7.) THE TIMESCALE OVER WHICH WE DECARBONIZE OUR ENERGY ECONOMY
DETERMINES THE SEVERITY, CONSEQUENCES, AND LONGEVITY OF FUTURE
WARMING.

The longer we delay meaningful climate action, the more severe the
consequences will get not just for all of humanity today, but for
generations and even millennia to come. Each time we add more of any
of these greenhouse gases to the atmosphere, they:

* increase the atmospheric concentration of that gas,
* which behaves as a blanket, increasing the atmospheric and surface
temperatures,
* where that gas and the heat mixes with the top of the ocean,
increasing its chemical concentration and overall temperature,
* which mixes with the deep ocean, distributing that chemical and
that heat throughout the oceans,
* where that extra oceanic heat leads to thermal expansion of the
ocean, icecap melting, and the destabilization of polar vortices,

among other effects. The longer that gas and that heat remains present
here on Earth, the more enduring and more severe the effects of
climate change are going to be. The ideal solution will be to
decarbonize our energy economy — overwhelmingly the top contribution
to greenhouse gases and rising temperatures — rapidly, thoroughly,
and sustainably. The worse we let things get, and the longer they stay
that way, the more difficult it’s going to be to mitigate, deal
with, and eventually reverse those downstream effects.

If zero carbon emissions were to be achieved today, temperatures would
level off and remain at their current elevated levels (blue line). The
scenario of constant concentrations, where temperatures continue to
rise, is not supported by the full suite of the interplaying systems
on Earth, requiring net emissions to maintain the current
concentrations (red line). (Credit: Carbon Brief/H. D. Matthews & A.
Weaver, Nature Geosciences, 2010)

8.) THERE IS HOPE: GLOBAL WARMING IS EXPECTED TO STOP ONCE NET CARBON
DIOXIDE EMISSION REACH ZERO.

As fast and severe as our current global warming trends are — with
temperatures increasing at an alarming rate of 0.21 °C (0.38 °F) per
decade — it isn’t a foregone conclusion that things must continue
to worsen very far into the future. If we start producing large
amounts of energy from sources that don’t emit large amounts of
carbon dioxide in the process, there’s reason for optimism. Once we
reach the point — and we all assume that we someday
will — where we achieve net-zero carbon emissions, global
warming will pretty much cease at that time.

Even though temperatures lag carbon dioxide concentrations, once we
reach that net-zero emissions goal, we can robustly predict that the
atmospheric concentrations of carbon dioxide will decrease, leading
to roughly level temperatures
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(And yes, temperatures will drop if we move to negative carbon
emissions.)

The Zero Emissions Commitment Model Intercomparison Project (ZECMIP)
is discussed thoroughly in the new IPCC report, and agrees with an
important 2010 paper [[link removed]]. The
conclusion is that as soon as we hit net-zero emissions, the
temperature will roughly be frozen-in at that value unless/until
negative emissions work to reverse the warming trend.

Earth energy budget diagram, with incoming and outgoing radiation
(values are shown in W/m^2). Satellite instruments (CERES) measure the
reflected solar, and emitted infrared radiation fluxes. The energy
balance determines Earth’s climate. (Credit: NASA)

All of this leads to an enormous question that humanity, as a
collective, is capable of determining the answer to: what will the
hottest year of the third millennium be? If we rapidly decarbonize the
energy sector over the next decade or two — reflecting the scenario
outlined in the two low-emissions IPCC scenarios — we can expect
those maximum temperatures to occur soon, in the latter half of the
21st century. If we follow a slower decarbonization path, like the
SSP2–4.5 scenario, the hottest year will likely occur sometime in
the 22nd century.

But if we follow one of the more sobering high-emissions scenario,
where 2100-era emissions are greater than they are today, we could be
eventually looking toward

* global temperature increases in excess of 5 °C (8 °F),
* global sea level increases in excess of 6 meters (20 feet),
* global carbon dioxide concentrations that crest above the 800 or
even the 1000 ppm threshold,
* and a rise in global temperatures that continues for many
centuries to come.

It’s up to all of us, collectively, to quite literally clean up
humanity’s act. The price of civilization doesn’t have to be the
thriving, wild ecosystem of planet Earth. But if we’re not careful
in how we steward this world we all rely on for our survival, this
will go down as yet another chapter in human history where we knew
what needed to be done, but failed to act in that necessary fashion.

_ETHAN SIEGEL is a Ph.D. astrophysicist and author of "Starts with a
Bang!" He is a science communicator, who professes physics and
astronomy at various colleges. He has won numerous awards for science
writing since 2008 for his blog, including the award for best science
blog by the Institute of Physics. His two books "Treknology: The
Science of Star Trek from Tricorders to Warp Drive"
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the Galaxy: How humanity looked beyond our Milky Way and discovered
the entire Universe"
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available for purchase at Amazon. Follow him on
Twitter @startswithabang [[link removed]].
Support ETHAN SIEGEL and STARTS WITH A BANG (with benefits!) HERE
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