# Lecture 49: Climate System II

### Internal Radiative Forcing Results Mainly from Greenhouse Effect

• Trace gases in the atmosphere are radiatively active → Absorb energy in certain wavelengths
• These are the "greenhouse gases"
• They are mainly in the troposphere
• Where do greenhouse gasses come from?
• Some are naturally occurring (﻿$CO_{2}$﻿, ﻿$CH_{4}$﻿, ﻿$H_2{O}$﻿, ﻿$O_{3}$﻿ )
• Some are anthropogenic (CFCs)
• Some are naturally occurring but remobilized by human activity (﻿$CO_{2}$﻿, ﻿$CH_{4}$﻿ are of greatest concern)

### Composition of Atmosphere

• ﻿$N_{2}$﻿ (Most)
• ﻿$O_{2}$﻿ ( ﻿$\frac{1}{4}$﻿ )
• Variable gases (worried that we change the amount in the atmosphere)

### Solar NRG Powers Life

• The sun radiates NRG, mainly in the shorter wavelength portions of the electromagnetic spectrum (UV)
• In the terrestrial NRG cycle it is degraded and leaves earth as long wavelengths (IR)
• Incoming NRG measured in top of atmosphere is called Insolation

### Trace Gases in the Atmosphere Act as Selective Radiation Filters

• Longwave filter absorbs outgoing IR radiation, causes heat to be retained near earth's surface
• Greenhouse effect
• Makes life possible on earth
• Planet would be much colder without it

### What Happens when More Gasses are Put in and it Becomes More Effective?

• We'd burn, a blanket would be put on top of earth

### Greenhouse Gases Warm the Lower Atmosphere

• As the surface absorbs solar UV radiation it increases in temp and emits IR radiation
• Greenhouse gases absorb this outgoing IR radiation
• Water vapor, ﻿$CO_{2}$﻿, etc...
• After absorbing outgoing IR NRG, Greenhouse gases re-emit it into the atmosphere
• Some is lost in space
• Some is re-radiated downward, warming the troposphere

### Greenhouse Gases Vary in their Ability to Cause Warming

• Global warming potential (radiative forcing potential) → Relative ability of a given greenhouse gas to contribute to greenhouse warming
• ﻿$CO_{2}-1$﻿, everything is compared to ﻿$CO_{2}$﻿ (over its atmospheric life of 100 yrs)
• Methane is 25 times as potent as ﻿$CO_{2}$﻿ over a 100 yr period
• Nitrous oxide is 296x as potent as ﻿$CO_{2}$﻿

### Planetary Albedo is Important to Climate Regulation

• Ice is leaving, meaning less reflection, more absorption of heat
• Volcanism can play a role in warming and cooling

### External Radiative Forcing Results from Astronomical Variations

• Processes outside the earth system
• Variations in solar output (sunspot cycles etc..)
• Variations in earth's orbit
• These processes result in cyclical variations in insolation (solar energy reaching the top of Earth's atmosphere)
• We have no control over these processes

### Variations in Earth's Orbit

• Axial wobble (E wobbles side to side when it orbits, so different parts face sun at diff times)
• Tilt (E is tilted, and the tilt varies between 20 - 25 degrees
• Variation in orbits (sometimes more circular, sometimes more elliptical
• Orbital variations cause cyclical variations in insolation and therefore in climate, including glacial cycles

Perihelion - Closest earth is to orbiting to sun

Aphelion - Furthest earth is to orbiting the sun

Climate varies naturally because of these influences, on variety of time scales.

### The Climate System is even more Complicated, because of Feedbacks

#### positive feedback cycles (self-reinforcing)

• Example → Warming ﻿$\rightarrow$﻿ evaporation of seawater ﻿$\rightarrow$﻿ more water vapor in atmosphere ﻿$\rightarrow$﻿ more warming

#### negative feedbacks

• Example → Warming ﻿$\rightarrow$﻿ evaporation of seawater ﻿$\rightarrow$﻿ more cloud cover ﻿$\rightarrow$﻿ blocks incoming radiation ﻿$\rightarrow$﻿ cooling

#### these feedbacks interact in complex ways

• Example → Aerosols and clouds can have either a warming or cooling effect

It is full of positive and negative feedback's, who are very opposite, these both pull on each other which one wins is our question we must figure out.

### Direct Atmospheric Sampling tells us about Present Atmospheric Conditions

• This hasn't been done long so we can't really look at trends and future
• Some new tech allows us to measure a lot easier

#### Tools for the Study of Climatic Change

• Historical (observed, measured) records of climate (have been kept since 1860s)
• Paleoclimatology → Study of ancient climates
• Rock record and fossil assemblages
• Climate proxies
• Natural events that are controlled by (and closely mimic) some aspect of climate
• Records kept either by people or by nature