GEO011 Midterm 2

Climate Sensitivity

Amount of warming that we expects to occur when there is a change in the factors that control climate (e.g., increase in GHGs)

Best estimate of climate sensitivity

3.0 C (5.4 F)

According to IPCC AR5, what is the range of climate sensitivity?

1.5 to 9.0 C (2.7 to 16.2 F)

Where do estimates of climate sensitivity come from?

1. Climate Models compare observed temperature changes from the instrumental record from the past 160 years.

2. Deep ocean temperature: Over the past few decades, it has also been possible to make use of temperature measurements from the deep oceans. While such data are useful, they are limited by an even shorter available record.

3. Climate Proxy Data/Paleoclimatic Data (tree rings, ice cores, etc.), we can estimate how the Northern hemisphere temperature varied during the past centuries.

4. Clues from Deeper Time: Geologists estimate that ancient atmospheres contained as much as 1500 ppm CO₂ (and possibly more).

What causes the delay in warming in response to a doubling of CO2?

Equilibrium Climate Sensitivity takes into account the fact that the full amount of warming in response to an increase in GHGs may not be realized for many decades, due to delayed ocean warming.

Why are future projections of climate change uncertain? Or more specifically, what are the two main sources of uncertainty?

1. Unknown trajectory of future GHG emissions.

2. Uncertain response of the climate to these emissions

How can scientists draw certain conclusions?

1. Best-guess scenarios of fossil fuel burning

2. Average projections of climate models

How are sunspots related to solar intensity? In modern climate, are there more sunspots (a brighter sun) than those (that) in the Little Ice Age? (2A.p13)

1. Sunspot records are available from the early 17th century through today.

2. More (less) sunspots = brighter (dimmer) sun

How was the Last Glacial Maximum (LGM) different than today? Why is the LGM cooler than today? How were atmospheric concentrations of greenhouse gases (GHGs) different? Sea level? Ice sheets? Earth’s orbital configuration?

1. CO2 content ~50% of what it is today (~180 ppm). Methane (Nitrous Oxide) was about ⅕ (⅔) of what it is today.

2. Vast ice sheets covered much of Canada, northernmost US, Scandinavia, and Europe. About half of the cooling is due to the corresponding increase in reflectivity.

3. Earth’s orbital configuration was also different > summer sunlight at high northern latitudes was reduced > winter snow could survive the summer > additional ice accumulated.

How has CO2 varied over geologic time scales? Has it varied a lot, or a little?

1. CO2 levels were high 500 million years ago, then fell, reaching a minimum 300 million years ago at the height of the Permo-Carboniferous glaciations on Gondwana.

2. After that, levels rose and fell, reaching another maximum 175 million years ago in the late Triassic.

3. They stayed relatively high through the next 100 million years ago (age of the dinosaurs > reptiles lived in the Arctic)

4. Since then CO2 has fallen, reaching another minimum close to present day.

5. Combined with paleo-temperature data, proxy CO2 data provide a climate sensitivity of 2-5 C (4-9 F) for each doubling of CO2.

What do fossil-fuel emissions scenarios consider? 

Driving forces of consumption are highly complex involving population growth and per-capita energy demand.

These factors are in turn closely linked to economic growth and technological advances that accelerate consumption and shift it to other climate-neutral sources.

How are Representative Concentration Pathways (RCPs) defined?

From the 5th assessment by the IPCC defines four scenarios based on their “total radiative forcing” by 2100: RCP 2.5, RCP 4.5, RCP 6. And RCP 8.5. 

What is meant by a “business-as-usual” scenario? What is RCP8.5?

Business-as-usual scenarios > Increasing fossil fuel use (RCP8.5)

A conservation/mitigation scenario? RCP2.6?

Conservation/mitigation scenarios > Future reduction of fossil fuel use (RCP2.6)

What is the “faux pause”? Can it be simulated by most model simulations? 

1. Temperatures haven’t increased as rapidly over the past decade as they did in the prior few decades has led to the false notion that there is a “pause” in global warming.

2. Not simulated by most model simulations.

What cause “faux pause”?

1. Background volcanic activity,

2. Short-term reduction in solar output

3. Series of La Niña events, all of which have led to temporary surface cooling not taken into account in most simulations.

Is it truly indicative of a pause in global warming?

Not a pause. 

By other measures in particular, the increasing heat content of the oceans, and the accelerating loss of Arctic sea ice, climate change and global warming are unfolding on, or ahead of, schedule.

How well did past IPCC projections do?

Quite Well

1. Actual CO2 increases since 1990 (FAR) have tracked roughly in the center of the projected range. 

2. Although the globe has warmed up a bit less than model projections (due to natural factors), projected warming is consistent w/ observations.

3. Sea level rise projections have fallen in each successive IPCC assessment.

4. Projections are consistent with the observed acceleration (as more land ice melts).

5. Substantial melting of ice sheets has begun earlier than expected upper end of projected range has been revised upward in AR5.

Does IPCC overstate the effects of climate change?

1. Opposite appease to be true

2. Decreasing trend of Arctic sea ice > observes decline is larger than model projections.

Under RCP8.5, how much warming does IPCC project by 2100? Note the warming magnitude relative to 2000 and relative to preindustrial. (2A.p36)

1. 2.5–4.6ºC (4.5–8.3ºF) (about 3.8–6.8ºC or 6.8–12.2ºF relative to pre-industrial time) for the least conservative, i.e., “business-as- usual” scenario (RCP 8.5 in figure below).

Under RCP2.6, how much warming does IPCC project by 2100?

0.2–1.8ºC (0.4–3.2ºF) (about 1.2–2.8ºC or 2.2–5.0ºF relative to pre-industrial time) for the most conservative emissions reduction scenario, involving strong mitigation (RCP 2.6 in figure below).

Is the surface warming spatially uniform? What are the warming patterns and reasons? (2B.p8)

1. Greatest warming will take place over the polar latitudes of the northern hemisphere, due to the positive feedbacks associated with melting sea-ice.

2. Greater warming is projected for land masses than for ocean surfaces, due mostly to the fact that water tends to warm or cool more slowly than land.

3. Weak North Atlantic warming due to change in ocean currents (weakening of Thermohaline Circulation)  

Which continent will see the most warming?

North America

How is precipitation projected to change? Why?

Projected poleward shift of jet stream may cause: Increased winter precipitation in polar and subpolar regions; decreased summer precipitation in many mid-latitude regions.

Poleward expansion of the Hadley cell will cause decreased precipitation in subtropics.

Warmer atmosphere will cause increased precipitation near the equator. 

How are floods and droughts projected to change? Why?

Decrease summer precipitation + increased evaporation due to warming surface temperature > greater tendency for drought in many regions.

More evaporation, more precipitation > more vigorous cycling of water through atmosphere > more frequent, intense rainfall events and flooding for many regions. 

What is El Nino Southern Oscillation (ENSO)?

A natural irregular oscillation of the climate system, involving inter-related changes in sea surface temperatures (SSTs), currents, and winds across the tropical Pacific.

How does the tropical Pacific change during El Nino?

Trade winds in the eastern and central tropical Pacific weaken or even reverse, there is little or no upwelling of cold sub-surface ocean water in the eastern equatorial Pacific, and warm water spreads out over much of the tropical Pacific ocean surface.

How does the tropical Pacific change during La Nina?

Trade winds in the eastern and central tropical Pacific are stronger than usual, and there is strong upwelling of cold, deep water in the eastern and central equatorial Pacific.

How long will it alternate between El Nino and La Nina?

This phenomenon alternates every few years (2-7 years, average 4 years)

How will El Nino affect global patterns of temperature and precipitation?

1. Weakening/reversal of southeast trade winds

2. Warmer water in the eastern tropical Pacific

3. Reduction in cold upwelling off coast of South America

4. Wetter conditions in southern United States

What are the characteristics of ENSO projections (climate overall? ENSO magnitude?)

Most (but not all) model predict a more El Nino like pattern (x-axis); Equal chance of increase/decrease in ENSO magnitude (y-axis)

Are ENSO projections certain?

Precise regional climate change projections are hampered by uncertainties in how some global winds and ocean currents will change, most notably ENSO.

What are the main reservoirs of carbon and processes of carbon transfer in the carbon cycle?

Atmosphere, Ocean, and Vegetation, Soils, and Detritus on Land

Is photosynthesis important? Respiration?

Various processes transfer carbon between these reservoirs, including photosynthesis, respiration, ocean–atmosphere gas exchange, and ocean mixing.

Does all of our CO2 emission stay in the atmosphere? If not, where does it go?

1. 55% of the CO2 we’ve pumped into the atmosphere since 1750 has “disappeared” > much has dissolved into the ocean, and the rest is incorporated into living biomass via photosynthesis.

2. 45% of the CO2 that has not “disappeared” but accumulated in the atmosphere is termed the “airborne fraction.” 

3. Nature has already responded to fossil-fuel burning and somewhat reduced the human impact on atmospheric composition and climate. 

4. But nature has its limits and humans are beginning to push up against them.

Land Biosphere Carbon Cycle

Positive Feedback: A warmer land leads to increased growth and respiration (CO2 byproduct) of soil microorganisms > carbon in soils is now being converted to CO2 at increasing rates. 

Negative Feedback: Plants increase their growth in response to elevated CO2 (“CO2 Fertilization”).

Ocean Carbon Cycle

1. Positive Feedback: A warmer ocean has less ability to absorb atmospheric CO2

2. Negative Feedback: Ocean acidification reduces the production of calcium carbonate (CaCO3; limestone) by organisms (corals, plankton).

   1. When these organisms grow their CaCO3 skeletons (i.e., to calcify), CO2 is released to the water. So calcification reduces the ocean’s ability to take up fossil-fuel CO2. 

   2. Thereby, less calcification will increase the ocean's ability to absorb atmospheric CO2. 

Biological Pump Feedback

(Positive Feedback) CaCO3 is relatively dense & acts as a ballast once as an organism dies, carrying its tissues to deep ocean > this “pump” of carbon removes CO2 from surface waters, allowing more CO2 to be absorbed.

• Loss of the ballast reduces the ocean’s ability to take up atmospheric CO2.

Biological Pump Feedback (Sluggish Ocean)

(Positive Feedback) A slowing of ocean circulation reduces the mixing of nutrients:

• Slows biological productivity

• Weakens biological pump > reduces ocean’s ability to absorb CO2.

Rock Weathering

(Negative Feedback) Increased temperature and rainfall stimulate the chemical weathering of rocks (the process that turns rock into soil and into dissolved salts in rivers). 

1. Atmospheric carbon dioxide dissolved in rain forms carbonic acid, which aids the rock-weathering process during which CO2 is converted to other forms of dissolved carbon. 

2. Increased weathering (during warming), therefore, removes CO2 from the atmosphere.

Which type of feedback dominates?

Positive (more warming) and negative (reduced warming) feedbacks are involved, but positive feedbacks prevail > Warming is reducing nature’s ability to absorb atmospheric CO2.

What does this imply for future climate change?

1. Models that simulate both the carbon cycle and climate have been run with some, but not all, of these feedbacks.

2. Overall effect of these carbon cycle feedbacks is a more rapid buildup of atmospheric CO2, and a warmer climate. 

How is sea level projected to change by 2100? Why?

Sea level is predicted to rise between 0.5-1.2 m (1.6-3.9 ft) by 2100 for two reasons:

1. Water becomes less dense (it expands) as it warms. A small rise of 0.1–0.4 m (0.3–1.3 ft) is predicted by 2100 due to this effect alone.

2. Melting of continental ice (not sea ice) including mountain glaciers and permanent ice caps. Melting of glaciers and ice caps > adds ~0.5 m (1.6 ft) of sea level rise.

How much will sea level rise with an eventual melting of Greenland?

5–7 m (16–23 ft)

How much will sea level rise with an eventual melting of West Antarctic?

5 m (16 ft)

What are some of the uncertainties associated with IPCC projections of future sea level rise (e.g., crevices/moulins)?

1. Scientists are surprised by an unprecedented, widespread surface melt that was observed over the Greenland ice sheet in 2012.

2. State-of-the-art models do not account for some newly observed effects that scientists now believe could significantly accelerate the rate of melting. 

3. Crevices (“moulins”) > Crack/fissure in ice sheet > Meltwater to penetrate deep into the ice sheet and lubricate the base, allowing large pieces of ice to slide quickly into the ocean.

   1. Could lead to a far more rapid disintegration of ice sheets than predicted by any current models. 

Will sea level rise happen everywhere over the globe (note the rebound effect)?

Sea-level has risen, except in regions (eastern Canada) where rising of the coastline has occurred due to Earth’s slow rebound from the ice age. 

How does sea level rise affect land and Gross Domestic Product (GDP)?

1. Even with 1 m of sea level rise—which could occur by the end of this century—losses of land and GDP are sizeable.

2. Losses rise dramatically at 5 and 10 m melting of Greenland would ensure the former; melting of West Antarctic ice sheet would ensure the latter.

How is extreme weather projected to change?

1. As climate changes, it is likely that the frequency and intensity of extreme weather events will change.  

2. For frosts and heat waves, the science is fairly definitive and the changes are intuitive.

3. For certain extreme weather events, such as severe frosts and extended heat waves, the science is fairly definitive and the predicted changes are intuitive. 

4. The greater the amount of warming, the more pronounced these trends will be. 

5. Changes are predicted to vary regionally.

6. More difficult to determine how extreme weather events such as tornadoes, severe thunderstorms, and hail storms will change due to their small-scale nature/processes.

7. However, individual storms will likely be associated with more severe downpours and more frequent flooding due to the greater amount of water vapor that a warmer atmosphere can hold.

Severe frosts?

1. As temperatures warm, the probability of frosts (nights when temperatures dip below freezing) will decrease markedly.

2. The greatest decrease in frost days is likely to occur in regions such as interior North America and Asia, where winter temperatures have been traditionally the coldest.

Heat Waves?

1. Heat waves (very high temperatures sustained over a number of days) are likely to become more intense, more frequent, and longer lasting.

2. The greatest increase in heat waves is predicted to occur in areas such as the western U.S., North Africa, and the Middle East, where feedback loops associated with decreased soil moisture may intensify summer warmth.

Intense rainfall events and flooding?

1. Increases in the frequency of intense precipitation events and corresponding flooding are expected.

2. Due to more vigorous water cycle that will accompany a warmer climate, with greater amounts of moisture in a warmer atmosphere 

Dry Spells

While many regions are likely to become drier, scientists predict that even in those regions individual rainfall or snowfall events will become more intense, although longer dry spells will separate them. 

Hurricanes

• Recent trend toward more intense hurricanes in certain ocean basins, such as the tropical Atlantic basin, and that these trends closely mirror warming ocean surface temperatures

• Warmer SSTs, all other things being equal, is likely to fuel more intense tropical cyclones, with stronger sustained winds. 

• Combination of sea level rise and stronger tropical storms could pose a “double whammy” when it comes to damaging storm surges. 

• Model simulations indicate a likely shift toward the strongest (Category 4 and 5) tropical cyclones over the next century, given projected climate changes.

• Uncertainties exist: 

  1. El Niño events change wind patterns over the tropical Atlantic region in such a way as to create unfavorable conditions for tropical cyclone formation. And there is still uncertainty about how El Niño will change in response to climate change.

What are required to achieve lower stabilization targets?

• A broad range of CO₂ stabilization targets have been analyzed.

• The lower the stabilization target, the sooner peak emissions must occur to stabilize CO₂ at 550 (750) ppm, need to reach peak by 2040 (2080).

• Lower stabilization levels can be achieved only with lower peak emissions the 750 (550) ppm target allows CO₂ emission rates to increase by 100% (50%).

• All stabilization targets require sharp reduction in CO₂ emissions following the peak.  

  • Low stabilization levels require CO₂ emission rates to fall below the current rate within a few decades.

To stabilize CO2 level at 450, 550, 750 ppm, by which year we need to reach emission peak?

1. To prevent atmospheric CO₂ levels from exceeding 450 ppm, fossil fuel use must peak by 2020.

2. Even with this, global temperature increases an additional 1°C (1.8°F) during this century, or 2°C (3.6°F) since preindustrial times, and sea level rises by at least 0.5 m by 2100.

Under climate change, what are the threats to coastal and low-lying regions (e.g., Bangladesh, Southern Florida, Netherlands)?

Temporary flooding and Permanent inundation caused by rising sea level

With 6 meters of sea level rise, will most of New York City be submerged?

Yes

What will be human loss in the coastal regions that are even not inundated by higher sea level?

1. increased exposure to flood and storm damage

2. more intense coastal surges

3. altered patterns of coastal erosion

How does global warming affect air pollution? Why?

1. Smog is produced when emissions from incomplete fossil-fuel combustion react to produce pollutants.

2. One such pollutant is tropospheric ozone, a lung irritant that also damages crops, buildings & forests.

3. Warming accelerates ozone production & promotes air stagnation, leading to increased tropospheric ozone levels.

4. One model indicates that for every 1°C increase in global temperature, there will be 20,000 additional pollution related deaths.

What is an ecosystem?

An interdependent community of plants, animals, and microscopic organisms, and their complex physical environment.

List ecosystem boundaries delineated by climate.

1. Desert ecosystems in the subtropics

2. Tropical rain forest ecosystems near the equator

3. Tundra ecosystems near the poles

Why are ecosystems valuable?

All these different elements interact to form a complex whole, with unique properties particular to that combination of living and non-living elements.

How does climate change affect ecosystems? Biodiversity?

1. As climates have changed in the geologic past, ecosystems have shifted in response.

2. But past climate changes were slower than the projected future changes human activity is already pushing ecosystem resilience, and may lead to species extinction if they cannot adapt or migrate fast enough.

According to the IPCC, what is the most vulnerable ecosystem? Why?

IPCC concludes that “coral reefs are the most vulnerable marine ecosystem with little scope for adaptation”. 

“Coral Bleaching”?

The loss of algae that live in a symbiotic relationship with the coral & give it color —is linked to exceptionally hot ocean temperatures.  Human induced global warming will contribute to this.