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The Future of the Earth’s Habitable Zone

Goals• Review the concept of a habitable zone• Introduce the concept of “continuously habitable zone”• How will the Earth’s habitability end?• Global climate change

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The habitable zone (review)

The range of distances around a star, at which a planet could potentially have conditions that would allow for abundant amounts of liquid water on the planet’s surface.

Venus: Earth’s twinM=0.815×MEarth

R=0.949×REarthD=0.723 a.u.

Atmosphere: 90 atmComposition: 96% CO2Temperature: 470ºC; 880ºF (Not 35ºC; 95ºF)

What would happen to the Earth, at Venus’ position in the solar system?

Temperature rise increased oceanic evaporation;Increased atmospheric H2O increased greenhouse effect;((Repeat, repeat, repeat))

The Earth becomes a new Venus.

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Our Sun’s habitable zone (Review)Inner boundary

– Determined by a runaway greenhouse effect;– Somewhere between 0.7 a.u. (Venus) and 1 a.u. (Earth).

– Standard models suggest the boundary ~ 0.84 a.u.– “Moist greenhouse effect” models suggest ~ 0.95 a.u.

Outer boundary– Determined by the greenhouse effect’s ability to keep water in

liquid state, despite being so far from the Sun;– Somewhere between 1.0 a.u. (Earth) and 1.5 a.u. (Mars).

If Mars had a stronger greenhouse effect (perhaps as in the past), it would be within a habitable zone limit ~ 1.7 a.u.

But, some models suggest CO2 snow would rob a planet of its CO2 atmosphere at distances exceeding ~ 1.4 a.u.

Summary: our Sun’s habitable zone is0.95 —1.4 a.u. -or- 0.84 — 1.7 a.u.

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Mars and the habitable zone

Just because a planet is habitable one day, it will not necessarily be habitable tomorrow!

Consider Mars—the evidence is extremely strong that it has known liquid water on its surface:

– Weathered features as viewed from space;– Carbonate deposits on the Martian meteorite suggesting pulses of

water flows;– Features such as hematite blueberries as detected by rovers.

The hematite blueberries suggest Mars was continuously habitable over long periods of time.

Mars is no longer habitable because it cooled, solidified, lost its magnetic field, and was solar-stripped of its atmosphere.

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Solar brightening: our evolving sun

The Sun is not the unchanging energy source you may think it is.

The Sun supports itself against inexorable gravity by energy-releasing nuclear reactions in its core:

1H + 1H + 1H + 1H 4He + ENERGY

Over time, the number of particles in the Sun’s core decreases….

…With fewer particles in the core, they have a harder time fighting gravity…

…The result is that the core compresses; in the process, it heats up…

…and the hotter, compressed core burns more furiously.

The Sun is about 30% brighter now than it was when it formed

Furthermore, it will continue to brighten into the future!

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Continuously habitable zone

As the Sun brightens over time, its habitable zone slowly changes—both edges of the habitable zone are moving away from the Sun.

The continuously habitable zone is the range of distances from the Sun that have been continuously habitable since the Solar System’s creation.

Estimates of the continuously habitable zone depend upon our assumptions about habitability, and the solar brightening.

What about us—what about the future?As the Sun gets hotter, the habitable zone of the solar system will move outwards, away from the sun.

We will be driven from the Earth. Where will we go?

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The ultimate end of our habitable zoneEvent Timescale Escape to…

Solar brightening 0.5-3 BY Mars, interplanetary space

Red giant phase (1000Lsun) 5 BY Interplanetary(Earth is 700ºC, 1300ºF) space

Planetary nebula phase 5+ BY Other stars(Earth probably vaporized)

Milky Way Galaxy exhausted 50 BY K, M stars(star formation ends)

K, M stars die 100 BY ??

Black holes evaporate; ?? ---Protons decay;Universe a sea of dilute photons

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Global Climate Change

– Evidence for “global warming”– Connection with greenhouse gases– Is the greenhouse gas emission anthropogenic?– Potential consequences– Uncertainties

Frequent complaint

“We can’t predict the weather for this week—why are you saying we can predict the weather for years to come?”

Weather is not climate. Climate is saying, “Florida is hotter than New York; Arizona is more arid than Seattle” This, we can predict.

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Evidence for global climate change

1. Data must be long-baseline;

2. Data must be obtained consistently;

3. Data should be insensitive to short duration effects.

Statements that are not scientifically significant include:– Wow, this year is really hot!– Wow, this year is really rainy!– Wow, the last two years we’ve had bad hurricanes!

Longer baselines are needed to justify such statements.

Huge ice deposits (such as glaciers or arctic/antarctic ice fields) help monitor overall global temperatures because…

– They have large thermal inertias, making them insensitive to minor yearly variations.

– They are made of water, which is (on Earth) near the cusp of being liquid vs. solid.

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Evidence for global climate change

Time to take a tour! Start by leaving Sewell

Destination #1 Pederson Glacier; Kenai Fjords National Park, Alaska.

From 1920 to the current, we have seen the recession of glaciers and the evaporation of the lagoon at the base of the glacier.

Destination #2Muir Glacier; Glacier Bay National Park & Preserve, Alaska.

The glacier has retreated by 20 km since 1941, this involved the melting of glacier ice 65 m thick.

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Evidence for global climate change

Destination #3 Greenland.

The Greenland ice sheet is melting faster than it grows, and is therefore shrinking.

Destination #4Larsen B Ice shelf; Antarctica.

The Larsen A ice shelf broke up in 1996, the Larsen B ice shelf broke up in 2002 (over 3 weeks); Larsen B had an area of Rhode Island.

Its speed of break up has astonished and thrilled scientists.

And on and on and on…a long parade of additional examples is found planetwide.

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Connection with greenhouse gasesWe know from studying Venus, Earth, and Mars, that the greenhouse effect is active and important.

Without the greenhouse effect, the Earth would be -18ºC (0ºF), but with it the Earth is about 14ºC (57ºF).

We know the gases that contribute to the greenhouse effect on the Earth1:H2O: 36-72%CO2: 9-26%CH4: 4-9% O3: 3-7%

Other gases include N2O and CFCs.

H2O vapor is rapidly regulated by the planet’s climate. CO2 is slowly regulated by the long CO2 cycle.

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Connection with greenhouse gases

Global climate change is an issue involving four factors. Do we see…

Remember that the greenhouse effect is enormous (32ºC, 57ºF) and unquestioned.

Global climate change asks whether the greenhouse effect is changing. The change being discussed is tiny compared to the massive greenhouse effect.

1) Changes in global temperatures?

2) Changes in concentrations of greenhouse gases?

3) Correlations between global temperatures and greenhouse gas concentrations?

4) Contributions to the greenhouse gas concentrations by our activities?

1414

Global temperatures and greenhouse gases

1) Indications of changes in global temperatures150 years worth of land-water temperature measurements were analyzed by NASA scientists.2 Note the observed increase in temperature since 1920.

2000 years of temperatures, inferred from ice cores, tree rings, pollen, sediment measures, etc., record climate events such as the medieval warm period and little ice age.3 These measures also indicate that the recent temperature increase is greater than in any other measured time.

2) Indications of changes in greenhouse gas abundancesIn Hawai’i, on the northern slope of Mauna Loa at 3400 m (11,000 feet), probes are recording atmospheric CO2 measurements.4 This overall rise is called the Keeling curve, and is observed at other research stations, too.

The tiny periodic changes are due to CO2 uptake by plants (northern forests dominate this, so the CO2 drops during the northern hemisphere summer).

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Global temperatures and greenhouse gases

3) Indications of a correlation between temperatures and greenhouse gasesBlue data—temperatures inferred from deuterium abundances at the Antarctica research site called Dome C.

Green data—Temperatures inferred from deuterium abundances at Lake Vostok.

Red data—Global ice volume (hence, global temperatures) inferred from 18O isotopes, related to fractionation from planktonic sediments.

Note the agreement of the three temperature measures with each other.

CO2 data at bottom—ice core data from Lake Vostok (blue), Dome C core (green), Law Dome (red), Siple Dome (cyan); atmospheric Mauna Loa (black).

Note the excellent correlation between temperature and CO2 concentrations.4, 5

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Global temperatures and greenhouse gases

4) Indications of contributions to the abundances of greenhouse gases by humansNote that the global levels of carbon emission have risen steeply, since around 1920.5

Recall 1920 is the approximate date at which we saw global temperatures rise in the 150-year graphic.

A regional breakdown of CO2 indicates that we are a chief culprit.6

Expect to see China’s contribution become increasingly important.

Note that it is easy to complain about the “Amazon rainforests being burned” when it is someone else’s error!

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Politics of global climate change

– Climate change will have economic impacts;– Scientists urge the precautionary principle;– 1997 Kyoto protocol to reduce emissions 5.2% from 1990 levels; USA did not sign;– Will costs of prevention outweigh costs incurred from climate change?

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Potential future consequences

Polar regions– The arctic should see 3-8ºC increase this century. Seasonal

ice sheets will diminish.– Expect a population collapse in polar bears and other arctic

species. (The polar bear is a keystone species.)

Albemarle Peninsula– Globally, the sea level is rising 0.3-0.5 cm per year. The

shoreline of North Carolina is changing: freshwater environments are becoming saltwater marsh.

– Thousands of acres have been lost, 50 cm of sea level rise will result in 750,000 acres being lost.

Florida keys– Sea warming contributes to coral bleaching. Reefs around

the world are dying.– Expect extraordinary losses of marine biodiversity.

1919

Other potential future consequences

Changing ranges of diseases

This is possibly a shell game.

More severe storms

The Atlantic storm intensity has doubled during the last 30 years.

More extreme climate variations

Californian impacts

Increased winter precipitation but decreased snowpack;

Increased summer temperatures;

Dryer summers (without snowmelt waters);

Increased wildfires.

2020

Sources cited1Kiehl, J. T.; Kevin E. Trenberth (February 1997). “Earth’s Annual Global Mean Energy Budget”. Bulletin of the American Meteorological Society 78 (2): 197-208.

2Hansen, J., Mki. Sato, R. Ruedy, K. Lo, D.W. Lea, and M. Medina-Elizade (2006). “Global temperature change”. Proc. Natl. Acad. Sci. 103: 14288-14293.

3Rhode, R.A. (2011). “Global Warming Art”. http://www.globalwarmingart.com/wiki/Temperature_Gallery, accessed 2/2011.

4Rhode, R.A. (2011). “Global Warming Art”. http://www.globalwarmingart.com/wiki/Carbon_Dioxide_Gallery, accessed 2/2011.

5Rhode, R.A. (2009). Data from “Global Warming Art”. http://www.globalwarmingart.com/wiki/List_of_temperature_related_images, accessed 2/2011, modified by Rice, B.A.

6Marland, G., T.A. Boden, and R. J. Andres (2003). “Global, Regional, and National CO2 Emissions" in Trends: A Compendium of Data on Global Change. Oak Ridge, Tenn., U.S.A.: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy.”