Goals: 1.Apply systems dynamics concepts of stock and flow to Earth’s energy budget. dE/dt = I in – I out =I up +I down 2.Figure out the incoming solar radiation for Earth and other planets 3.Compare reflectivity of different parts of the Earth system, both on the surface and in the atmosphere. 4.Predict the impacts of altering solar energy or reflectivity on flows of energy in Earth's climate system, and therefore Earth's temperature (left side of Earth's energy budget diagram) 5.Classify particular changes in incoming solar radiation and albedo as forcings or feedbacks. Day 4: Earth’s Radiation Balance – Incoming solar radiation and albedo

At the top of the atmosphere, a flying carpet 1 meter square directly facing the Sun, receives about 1360 Joules per second W/m 2  Solar Constant Since the Earth is spherical and it rotates, the 1360 W/m 2 gets spread out to an average of 340 W/m 2 received at the top of the atmosphere. Image credit: NASA

The Earth “blocks” an area in space equal to the area of a circle with Earth’s radius (  r 2 ) Each square meter of that imaginary circle gets 1360 W/m 2 If you were at the top of the atmosphere directly facing the Sun, you’d get 1360 W/m 2 The total W Earth captures is 1360 W/m 2 *  r 2 The AVERAGE square meter, with this total spread out over the whole spherical planet, is 1360 W/m2 *  r 2 /4  r 2 or 1360 W/m 2 divided by 4  about 340 W/m 2. (Aside: Trenberth was working with about 1365 W/m2 for the solar constant  341 W/m 2 )

How much does the solar constant vary?

Trenberth et al., 2009 Earth’s Energy Budget - Worksheet Where DOWN is Positive (+) And UP is negative (-)

80 Wm Wm Wm Wm -2 What is the heating (pos) or cooling (neg) rate of this layer, in Wm -2 ? a)-10 Wm -2 b)-20 Wm -2 c)+20 Wm -2 d)+50 Wm -2 e)+70 Wm -2

Surface Albedos NASA Composite of Earth’s surface if the planet were cloud-free

NASA Clicker Q: What if the forest expanded into the desert area (and nothing else changed)? Reflectivity would _______. NASA A. Increase B. Decrease C. Stay the same

NASA Clicker Q: What if globally sea level rose? Reflectivity would _______. A. Increase B. Decrease C. Stay the same

Some (mostly) Surface Albedos NASA From Compare among these

Thick clouds have more water droplets, if the size of droplets don’t change then increasing cloud thickness increases reflectivity (more reflective) NASA MORE reflectiveLESS reflective THICK: ref= THIN: ref= A warmer world will have higher water vapor concentrations, but it is possible that there will be larger areas where descending dry air evaporates clouds, making them thinner/less reflective. 11 Cloud Albedo – cloud reflectivity is one of the big unknowns that make predictions about future climate uncertain. Will future cloud albedos be bigger or smaller?

NASA MORE reflective LESS reflective The size of cloud droplets is determined by how thick the clouds are and how many particles are in the atmosphere for the cloud droplets to form on. In a polluted atmosphere, many droplets form on smog/haze particles, so that the average dropsize is smaller. More drops means more surface area for reflection, and brighter clouds, given the same cloud thickness. But … both cloud droplet size and cloud thickness change together, which is makes future cloud reflectivity hard to predict 12 Droplet size also matters

Trenberth et al., 2009 “Forcings” versus “Feedbacks” One useful way of thinking about forcings versus feedbacks in the climate system is to ask: Is it a process, or sequence of processes, that respond(s) to changes in Earth’s temperature and, in turn, influence(s) Earth’s temperature?  FEEDBACK Is it a process that can change Earth’s temperature, but does not, in turn, respond to changes in Earth’s temperature?  FORCING

Trenberth et al., 2009 Forcings and Feedbacks

1.Balances of flows of energy in, out, and within Earth’s climate system determine Earth’s temperature. 2.The energy Earth absorbs depends both on our distance from the Sun (which determines the solar constant) and on how much incoming solar energy gets reflected. 3.Different surfaces and atmospheric constituents have different albedos. 4.Changing solar energy or reflectivity, (or greenhouse gases – coming later), all alter the flows of energy in Earth’s climate system, and thus Earth’s temperature. 5.Climate “forcings” change temperature, but don’t respond to temperature changes. Climate “feedbacks” respond to temperature changes, and, in turn, influence temperature. 6.The Planck feedback is the most important ___________ feedback in the climate system. Summary