The Whole Earth Course Why the climate problem is difficult… but not impossible: Feedbacks & Forcing Instructor: Dr. Steven M. Lazarus Fall 2015 Science may be stimulated by argument and debate, but it generally advances through formulating hypotheses clearly and testing them objectively. This testing is the key to science. In fact, one philosopher of science insisted that to be genuinely scientific, a statement must be susceptible to testing that could potentially show it to be false (Popper, 1934). http://www.esrl.noaa.gov/gmd/ccgg/carbontracker/

EARTH: The operators’ manual Is the current climate unusually warm? EARTH: The operators’ manual Fig 9.1 IPCC AR4 Is the warming trend of the past 100 yrs unusual? What might be the natural causes of the recent (~100 yr) warming? IN vs. OUT Current anthropogenic contributions are about 130 X GREATER than volcanic sources!!! Short-Term COOLING Not much change! SLOW COOLING!

BP Planet Pages 408-410 positive feedback: ++++++++++++++++++++++++++ A system response in the direction of the initial forcing. This implies that the system is unstable (i.e. given some forcing the system moves away from equilibrium). negative feedback:--------------------------------------------- A system response in the opposite direction of the initial forcing. This implies that the system is stable (i.e. given some forcing, the system returns to equilibrium). In terms of climate, we use the term forcing to mean “radiative” In a broad sense, a feedback means that some fraction of the output is fed back into the input, so the radiative perturbation gets an additional nudge (amplifying the forcing, a positive feedback or damping the forcing, a negative feedback).

An equilibrium is considered stable (for simplicity we will consider asymptotic stability only) if the system always returns to it after small disturbances. If the system moves away from the equilibrium after small disturbances, then the equilibrium is unstable. If we perturb the climate with some forcing, what is the temperature change needed to allow the planet’s energy balance to be satisfied. For every Watt per square meter of radiative forcing the climate would warm by about 0.3 °C without any other responses. It would take about five doublings of CO2 or a 7% increase in the total solar radiation hitting the Earth to produce the magnitude of climate change typical of glacial-to-interglacial transitions.

What is so dang important about feedbacks? The function of feedbacks is to modify how much the climate (temperature) changes for a given amount of forcing. Strong evidence from ocean sediment data and from modeling links abrupt climate changes during the last glacial period and glacial-interglacial transition to changes in the Atlantic Ocean circulation. ~0.01 degrees C per century Temperature changes during IG-G transitions are accelerated due to feebacks! CO2, albedo, H20v.

Atmospheric Feedbacks (response) Water vapor feedback accounts for large warming predicted by climate models in response to an increase in CO2. + feedback Accounting for the ‘spectral overlap’ between the various GHGs and clouds, water vapor makes up roughly 50% of the modern GHE, clouds ~25%, CO2 ~20%, & methane, ozone, and nitrous oxide most of the rest Fig 11.15, pg 337 RESPONSIBLE FOR A DOUBLING OF THE EQUILIBRIUM TEMPERATURE IN RESPONSE TO CO2 FORCING!!! NON-LINEAR!

Ice-covered area Decreases Ice-Albedo Feedback Fraction of solar energy reflected back to space. + feedback Snow/ice albedo of about 0.7 to 0.9 Ocean albedo <0.1 Warmer climate  Shrinking Cryosphere Earth’s albedo decreases more solar energy would be absorbed. IN OUT Albedo Decrease Surface Warming Ice-covered area Decreases Albedo decreases Absorption of Solar Flux Increases In addition to water vapor, this mechanism is thought to be one of the primary reasons for the high latitude warming.

incr. absorption = warming incr. reflection = cooling Forcing: Anthropogenic Aerosols Small solid or liquid particles suspended in the atmosphere from industrial activity and biomass burning (sulfate aerosols). They reflect incoming sunlight (SW) and, to a lesser degree, absorb infrared (LW) radiation (direct effect). SW incr. absorption = warming incr. reflection = cooling absorption + reflection/scatter + transmission LW LW Perhaps more importantly, they also serve as condensation nuclei for clouds (indirect effect). SW more aerosols fewer aerosols many small drops = more reflective a few big drops = less reflective

COUNTERS THE WARMING CAUSED BY CO2!!! absorption reflection transmission increase NET aerosol effect: The thought is that the increased reflection of sunlight to space—both directly by the aerosols themselves and through their effect on increasing the reflectivity of clouds—outweighs any increase in their greenhouse effect, thus cooling the planet. COUNTERS THE WARMING CAUSED BY CO2!!! Unlike greenhouse gases, sulfate aerosols only remain in the atmosphere a few weeks before they are washed out by rain and snow—hence their abundance is proportional to their rate of production—as soon as production decreases, sulfate aerosols follow suit. ???

> 0 warming < 0 cooling indirect The understanding of anthropogenic warming and cooling influences on climate has improved since the IPCC TAR, leading to very high confidence that the effect of human activities since 1750 has been a net positive forcing of +1.6 [+0.6 to +2.4] W m-2.

? ? Summary Feedback Slide in new level + feedbacks dominate out Increase  Steady flow (CO2) new level What will be the new equilibrium temperature? How long will it take to reach equilibrium? Stabilization of CO2 at current emission levels would result in continued increase in atmospheric CO2 over the 21st century and beyond. + feedbacks dominate ? both + and - feedbacks temperature - feedbacks dominate time ?

So what do we know? Human activities are changing composition of Earth's atm. There is no doubt this buildup of carbon dioxide and other greenhouse gases is largely the result of human activities. It's well accepted by scientists that greenhouse gases trap heat in the Earth's atmosphere and warm the planet (radiative transfer models). A warming trend of about 0.6 °C (+/- 0.2 °C) has been recorded since the late 19th century (in both the northern and southern hemispheres, and oceans). Substantiated by melting glaciers, decreased snow cover in the northern hemisphere and…

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