Global Energy Balance and the Greenhouse Effect What determines Earth’s surface temperature? What is the history of CO 2 on Earth? ultravioletinfrared

Global Energy Balance The surface of the Earth will heat up until Energy in = Energy out Earth surface temperatures would be -18°C in balance with incident solar radiation if there were no greenhouse gases but greenhouse gases trap outgoing heat, so Earth’s surface will warm until enough heat is radiated through the top of the atmosphere to balance sun’s radiation Venus’ atmosphere is 96% CO 2 surface T = 450°C (850°F) Earth’s atmosphere is % CO 2 surface T = 15°C (60°F)

the atmosphere is transparent to most solar radiation but strongly absorbs Earth’s outgoing radiation SUNEARTH

So Earth surface receives “shortwave” solar radiation AND “longwave” radiation from atmospheric absorption of greenhouse gases. Earth’s surface will warm to the point that the net outgoing energy balances the net incoming energy. T surface = 298K even though T effective = 255K

So what stabilizes Earth’s long-term climate? Why has the Earth’s T remained within a relatively small range?

Factors controlling the weathering rate (drawdown of CO 2 ) Weathering = “Permanent” Removal of CO 2 from atmosphere: ~thousand-million years Volcanism = Addition of CO 2 to atmosphere: million years Factors controlling the addition of CO 2 to atmosphere Plate spreading rate Global volcanism

the key: weathering rates controlled by T, so higher T = more weathering = more CO 2 removed  T stabilized NB: balance only achieved over million-year timescales!

Atmospheric CO 2 over recent geologic time; changes caused by plate tectonics, volcanism Pre- industrial hard to get records of such old temperatures but evidence of crocodiles above Arctic circle 100Ma global T >30°C?

Ice core climate and CO 2 records tiny gas bubbles in the ice trap ancient air samples

Atmospheric CO 2 over very recent geologic time note that plate tectonics not responsible for these changes! small variations in Earth’s orbit (not Sun) drive Ice Ages glacial cycles: CO 2 +80ppm global average temperature = +5°C

ice core CO2 records confirm that the CO2 trend began in the 1800’s -clear land for agriculture -Industrial Revolution NOTE: it would take hundreds of years to remove the CO 2 we have put into the atmosphere so far

The ‘instrumental’ record of climate shows a ~1ºC warming over the last century

Why do % of climate scientists believe that CO 2 is warming the planet? 1.Theory predicts that increasing atmospheric CO 2 should warm the planet. 2.Geologic evidence links CO 2 and temperature in the past. 3.The warming is unprecedented in the most recent centuries (dwarfs natural variability). 4.Climate models show that rising CO 2 is necessary to simulate 20 th century temperature trends (solar and volcanic minor players).

To understand how climate has changed in the past, we need to use records of climate preserved in ice cores, ancient tree rings, coral bands, and other “paleoclimatic” sources: key is to CALIBRATE to temperature records

The “Hockey Stick” Key Points: error bars increase as you go back in time natural variability accounts for <0.5ºC over the last millennium late 20 th century temperature trend is unprecedented #3

Intergovernmental Panel on Climate Change (IPCC) 2001 #4