Weather, Climate & Society ATMO 325 Climate Projections

What is Climate Change? Climate change - A significant shift in the mean state and event frequency of the atmosphere. Climate change is a normal component of the Earth’s natural variability. Climate change occurs on all time and space scales. A plethora of evidence exists that indicates the climate of the Earth has changed. (Next Lecture)

What is Climate Change? If we can’t predict the weather more than 7-10 days in advance, why should I believe that we can provide useful 100 year climate outlooks? Weather forecasts attempt to predict specific weather at specific times for point locations. A climate “forecast” attempts to replicate changes in the statistics of weather. - Average temperature, rainfall, etc. - Distribution of weather events

Causes of Climate Change Atmospheric Composition - Anything that changes the radiative properties of the atmosphere (carbon dioxide, clouds, aerosols). Astronomical - Anything that alters the amount or distribution of solar energy intercepted by the Earth (solar variations, orbital variations). Earth’s Surface - Anything that alters the flow of energy at the Earth's surface or changes its distribution (snow cover, continental drift).

Global Climate Model: Take 2 A = albedo - % solar reflected to space (1-  ) emitted to space  = emissivity - % absorbed by air F OUT SFC F IN SFC (S 0 /4) AS 0 /4 (1-  ) F OUT SFC Surface T SFC Atmosphere T ATM  F OUT ATM Let ’ s model the Earth system as a planetary surface with an absorbing atmosphere above the surface.

(1-  ) emitted to space  absorbed by air F OUT SFC =  T 4 SFC (1-A) S 0 /4 (S 0 /4)AS 0 /4 (1-  ) F OUT SFC Surface T SFC Atmosphere T ATM   T 4 ATM  F OUT ATM =   T 4 ATM A planetary surface with an IR absorbing atmosphere above the surface. Global Climate Model: Take 2

1. An OLR absorbing atmosphere slows the net energy flow out from surface (relative to no atm). 2. An increase in atmosphere’s absorptivity causes surface T to increase. 3. Radiation reaching space from the Earth is a combination of emission from a warm surface and colder atmosphere. It must be equal to (1-A)S 0 /4 at equilibrium. 1-Layer Model Summary Courtesy J. Thornton UW

Global Climate Model: Take 2 1.The atmosphere absorbs only Outgoing Long wave Radiation (no absorption of solar radiation) 2. The atmosphere absorbs the same fraction of OLR at each wavelength. 3. The atmosphere has a uniform temperature. 4. F in = F out for each component and whole system. Simplifying Assumptions Courtesy J. Thornton UW

Global Climate Model: Take 2 By now, this is tattooed on your brain Courtesy J. Thornton UW

Global Climate Model: Take 2 If  ~ 0.75, then T SFC ~ 288 K Courtesy J. Thornton UW

How can you warm the T SFC ? 1.Increase the solar output S 0 2.Decrease the reflectivity A 3.Increase the absorptivity 

To increase T SFC by 1˚C… 1.Increase S 0 by ~20 W/m 2 2.Decrease A by ~1% 3.Increase absorptivity  by ~0.02

Can GHG increases explain warming? IPCC Fig. SPM.1 IPCC WG1 Fig. 6.10

IPCC SYR 1-1 Other changes are consistent with warming

CO 2 has increased from 290 ppm in 1900 to 380 ppm today

Evidence of Warming Temperatures have increased 0.6 K the past 100 years 0.6 o C warming past century Ahrens, Fig 13.5

Simple approach to GHG warming If a 33% increase in CO 2 could alone produced a 0.6 K warming, would a projected doubling of CO 2 the next 100 years produce a 1.8 K warming? Can it possibly be that simple? Not Really

Climate Feedbacks Feedbacks more than double the response of the temperature to increasing concentrations of greenhouse gases

Closer look at climate system reveals couplings between physical processes

Ice/Albedo Feedback Surface Temperature (+)(+) Snow and Ice Cover Planetary Albedo

Temperature/Water Vapor Feedback Surface Temperature (+)(+) Atmospheric H 2 O vapor Greenhouse Effect

Temperature/Low Clouds Feedback Surface Temperature (-)(-) Atmospheric H 2 O vapor Low Clouds Planetary Albedo

Temperature/Infrared Flux Feedback Surface Temperature Outgoing IR Flux (—)

Multiple feedbacks complicate the response of the climate system

Positive and Negative Feedbacks Assume that the Earth is warming. - Warming leads to more evaporation from oceans, which increases water vapor in atmosphere. -More water vapor increases absorption of IR, which strengthens the greenhouse effect. -This raises temperatures further, which leads to more evaporation, more water vapor, warming… “Runaway Greenhouse Effect” Positive Feedback Mechanism

Positive and Negative Feedbacks Again assume that the Earth is warming. - Suppose as the atmosphere warms and moistens, more low clouds may form. - More low clouds reflect more solar radiation, which decreases solar heating at the surface. - This slows the warming, which would counteract a runaway greenhouse effect on Earth. Negative Feedback Mechanism

Positive and Negative Feedbacks Atmosphere has a numerous checks and balances that counteract climate changes. All feedback mechanisms… Operate simultaneously. Work in both directions. The dominant effect is difficult to predict. Cause and effect is very difficult to prove at the “beyond a shadow of a doubt” level.

Synthesis Fig i-1

Observed distribution of temperature changes show warming over land IPCC SYR 1-2

Many sources are responsible for the increase in potent GHG’s IPCC SYR 2-1

Industrialized sectors are the biggest per capita contributors to GHG’s IPCC SYR 2-2

But underdeveloped sectors are among the biggest overall contributors per GDP IPCC SYR Fig 2-2

WG1 SPM.2 Attribution of RF changes ( )

WG1 SPM.4

IPCC SYR 3-1 GHG Scenarios Integrated world, fossil fuel emphasis, 2050 pop max Integrated world, balanced fuels, 2050 pop max Integrated world, non-fossil fuel emphasis, 2050 pop max Divided world, rapid unchecked pop growth Integrated world, more ecol. friendly, 2050 pop max Divided world, more ecol. friendly, slower pop rise

WG1 SPM.5 Divided world, rapid unchecked pop growth Integrated world, balanced fuels, 2050 pop max Integrated world, more ecol. friendly, 2050 pop max

WG1 SPM.5

WG1 SPM.6 Integrated world, balanced fuels, 2050 pop max Divided world, rapid unchecked pop growth Integrated world, more ecol. friendly, 2050 pop max

WG1 SPM.6 B1 A1B A2

WG1 SPM.7 Precipitation Changes (%)

IPCC SYR Fig. 3.5 Projections and model consistency of relative changes in runoff by the end of the 21st century

“Some systems, sectors and regions are likely to be especially affected by climate change.” “particular ecosystems: - terrestrial: tundra, boreal forest and mountain regions because of sensitivity to warming; Mediterranean-type ecosystems because of reduction in rainfall; and tropical rainforests where precipitation declines - coastal: mangroves and salt marshes, due to multiple stresses - marine: coral reefs due to multiple stresses; the sea-ice biome because of sensitivity to warming”

“Some systems, sectors and regions are likely to be especially affected by climate change.” “water resources in some dry regions at mid- latitudes and in the dry tropics, due to changes in rainfall and evapotranspiration, and in areas dependent on snow and ice melt” “agriculture in low latitudes, due to reduced water availability” “low-lying coastal systems, due to threat of sea level rise and increased risk from extreme weather events” “human health in populations with low adaptive capacity.”

“Uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic…” “Increasing atmospheric CO2 concentrations lead to further acidification.” “Projections based on SRES scenarios give a reduction in average global surface ocean pH of between 0.14 and 0.35 units over the 21st century.” “Progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species.”

IPCC SYR Fig. 3.6

Extreme Weather Events “Altered frequencies and intensities of extreme weather, together with sea level rise, are expected to have mostly adverse effects on natural and human systems.” IPCC Synthesis Report

Hurricane Intensities Increase by 10% Reason: Warmer Sea Surface Temps Increases in the frequency and/or the intensity of extreme weather events is a likely consequence of global warming. GFDL Super TyphoonGFDL Super Typhoon

99% 90% 66%

“Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change. “Partial loss of ice sheets.” “Rapid sea level rise on century time scales cannot be excluded.” “There is medium confidence that approximately 20 to 30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5 to 2.5 ｡ C (relative to ). As global average temperature increase exceeds about 3.5 ｡ C, model projections suggest significant extinctions (40 to 70% of species assessed) around the globe.” 5 in 10 chance

“Anthropogenic warming could lead to some impacts that are abrupt or irreversible, depending upon the rate and magnitude of the climate change. “It is very likely that the meridional overturning circulation (MOC) of the Atlantic Ocean will slow down during the 21st century. “Impacts of large-scale and persistent changes in the MOC are likely to include changes in marine ecosystem productivity, fisheries, ocean CO2 uptake, oceanic oxygen concentrations and terrestrial vegetation. Changes in terrestrial and ocean CO2 uptake may feed back on the climate system.” 90% chance

Key Points: CO 2 Warming CO 2 levels are rising and will likely double by The greenhouse relationship between higher CO 2 levels and warmer temperatures is indisputable. Even with perfect knowledge of future CO 2 levels, there is significant uncertainty about how much warming would occur and how fast it would occur. Model results suggest ~2 o C global warming, with strongest warming in polar regions, and an overall increase in global precipitation. Shifts in precipitation are much more uncertain, as are the consequences on water resources.

Intergovernmental Panel on Climate Change A consensus document of of a wide sampling of the scientific community IPCC Reports 2007 “Most of the observed increase in globally averaged temperatures since the mid-20 th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations” Available for download at