Unit VIII. Global WarmingA. 20th Century WarmingB. A perspective on 20th century warming. C. Causes of Global Warming

• Greenhouse gases• Solar variability• Volcanism• Aerosols• Clouds

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gasesa. CO2

Throughout the world, CO2 concentrations in the atmosphere have been increasing at a steady rate since we have been measuring them (1957).

From about 315 ppt to 370 ppt, or almost 20% in the past 50 years.

y = 0.1247x + 330.2

R2 = 0.96

320

325

330

335

340

345

350

355

360

365

370

375

380

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204 216 228 240 252 264 276 288 300

Series1

Linear (Series1)

12 months x 0.1247 = 1.5 ppm increase each year

1975 1998

First-order linear trendWhat is the second-order trend?

-15.00

-12.00

-9.00

-6.00

-3.00

0.00

3.00

6.00

9.00

12.00

15.00

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 180 192 204 216 228 240 252 264 276 288 300

Series1

1975 1998

Second-order trend

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gasesa. CO2

Second-order trend is the biosphere breathing….Largest variation is where seasonal contrasts are large and a large terrestrial biosphereSmallest variation where seasonal contrasts are small (tropics) and where there is little terrestrial biosphere. The Primary Productivity of the terrestrial biosphere is much larger than the ocean biosphere.

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gasesa. CO2

b. Methane (CH4)Also has been steadily increasing over the period of instrumental observation since 1983 (1600 to 1750 ppb).Strong seasonal cycle.

Sources and 100-year trend?

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gasesa. CO2

b. Methane (CH4)c. Other GHG (CFCs, NOx)

All GHG are thought to have had steady increases over the past 100 years.

Can this explain Global Warming?

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gases2. GHG and global warming.

Because roughly half of the observed warming occurred before significant anthropogenic GHG had been added to the atmosphere, and then warming ceased, even though even more GHG were added, GHG alone cannot explain all of the observed 20th century warming.

Unit VIII. Global WarmingC. Causes of Global Warming

3. Longer term changes in GHG.Ice cores preserve bubbles of trapped atmosphere from the time the snow fell.

The amount of CO2 in these bubbles is an accurate measure of CO2 levels in the atmosphere when the snow fell.

We can extend the instrumental record of CO2 into the distant past.

280 ppm

Pre-industrial

Holocene(present interglacial or warm time)

Glacial period

Unit VIII. Global WarmingC. Causes of Global Warming

3. Longer term changes in GHG.

Ice cores show that CO2 levels in the present interglacial were always 260 to 280 ppm until the Industrial Revolution.

And they were much lower (180 to 200 ppm) during the last glacial period.

Can we see any farther back?

450,000-year record of atmospheric CO2 from Vostok Ice Core

Interglacials

Unit VIII. Global WarmingC. Causes of Global Warming

3. Longer term changes in GHG.Can we see any farther back? Yes!

Current levels of atmospheric CO2 , CH4, CFCs and NOx are much higher than at any time in the past 450,000 years.

Unit VIII. Global WarmingC. Causes of Global Warming

1. Greenhouse gases2. GHG and global warming.3. Longer term changes in GHG4. Solar variability

a. Long-term changes in the solar constant. The Sun has been getting brighter since the Earth was formed, 4.6 billion years ago.But these changes are not detectable over the past 200 years.

Unit VIII. Global WarmingC. Causes of Global Warming

4. Solar variability a. Long-term changes in the solar constant b. Changes due to irregularities in Earth’s

orbital parameters. Precession of the equinoxes 21,000 yearsTilt of the spin axis 41,000 yearsElliptical orbit 100,000 years

Milankovitch Effect….. Also not significant on 200 year window.

Unit VIII. Global WarmingC. Causes of Global Warming

4. Solar variability c. Systematic variations in the “solar

constant”. There is an 11 year cycle in solar irradiance. This is in some way related to the 22 year cycle in the Sun’s magnetic field reversals.

Over the 11-year cycle, solar irradiance varies by about 0.1%.

0.1%

c. Systematic variations in the “solar constant”. There is a link between the systematic

variations observed in solar irradiance and the number of sunspots, dark spots visible on the Sun. Sunspots are dark areas on the Sun. But, these dark spots are surrounded by larger brighter areas.

There are even earlier observation of sunspots, dating back more than 2000 years, but the oldest reliable maps come from Galileo in the 1600s.

Accurate observations of sunspot cycles restricted to the last 150 years. There appears to be a smaller number of sunspots in the early 1900s.

4. Solar variability c. Systematic variations in the “solar constant”. Sunspot number as a proxy for solar irradiance. 11 year Sunspot Cycles: Maximum number of sunspots = 0.1% more irradiance than no sunspots. Maunder minimum (1640 - 1720 AD) coincides with the start of the Little Ice Age.Late 20th century has the highest number of sunspots in 500 years.

C. Causes of Global Warming5. Volcanism in the climate system

What is required?• Explosive volcanism injects material into

the stratosphere, where it has a relatively long residence time.

Fluid volcanism injects things into the troposphere, where precipitation scrubs it out within days to weeks.

• Need to inject stuff that impacts climate.What is this stuff?

C. Causes of Global Warming5. Volcanism in the climate system (Laki)

• Explosive volcanism • Need stuff that impacts climate.

1) Particulates: Residence time in troposphere is days to weeks, scrubbed out by precipitation and gravity.

Even in stratosphere gravity removes in weeks to months.

C. Causes of Global Warming5. Volcanism in the climate system (Laki)

1) Particulates: Volcanic Ash2) Aerosols: tiny airborne particles of matter,

either liquid droplets or solids, that are so small that they remain suspended for a long time. Volcanos emit large volumes of sulfur dioxide (SO2) gas. SO2 reacts quickly with oxygen and water vapor in the atmosphere to form tiny droplets of sulfuric acid.

Sulfuric acid aerosols reflect and scatter incoming solar radiation, reducing the energy received at the Earth’s surface. Consequently, they cool the surface.But, they are also greenhouse gases, and absorb Earth’s outgoing long-wave energy.Consequently, they warm the surrounding atmosphere.

5. Volcanism in the climate system (Laki)1) Particulates: Volcanic Ash2) Aerosols:

Residence times?In troposphere, sulfuric acid comes out with rain

(acid rain) in days to weeks.In stratosphere it remains for 1 to 3 years.

Only explosive volcanism can inject materials into the stratosphere, where there residence time is long enough to impact climate.But not for more than 1 to 3 years.

C. Causes of Global Warming5. Volcanism in the climate system (Laki)

• Explosive volcanism • Need stuff that impacts climate.• Pinatubo as an example: Explosive

volcanism, Reached >35 km, well into the stratosphere, Predicted…, 30 million tons of SO2 transferred to the stratosphere. Satellites allowed real-time monitoring of

impacts.

C. Causes of Global Warming5. Volcanism in the climate system (Laki)

• Pinatubo allow us to quantify the perturbations to climate due to past explosive volcanism. What do we predict?

• Less solar energy reaching the Earth’s surface.• Planet will cool.• GHG effect warms stratosphere• Duration: 1-3 years

C. Causes of Global Warming5. Volcanism in the climate system

• Pinatubo as an example

What about the stratosphere?Explosive volcanism can simultaneously cool the surface and troposphere, but warm the stratosphere.

Stratospheric temperature changes 1979-1994

Unit VIII. Global WarmingC. Causes of Global Warming

6. Aerosols and cloudsAerosols: tiny airborne particles of matter, either liquid droplets or solids, that are so small that they remain suspended for a long time.

In all cases, residence time is much longer in the stratosphere than in the troposphere, where most aerosols are scrubbed out by rainfall within weeks to months.

C. Causes of Global Warming6. Aerosols and cloudsAerosols: tiny airborne particles of matter, either liquid droplets or solids, that are so small that they remain suspended for a long time.• acid droplets (sulfuric, hydrochloric)• very fine desert dust• sea salt spray• sulfate aerosols (also from combustion)• fertilizers • soot: tiny particles of black carbon. Derived from incomplete combustion.

Unit VIII. Global WarmingC. Causes of Global Warming

6. Aerosols and cloudsHow do aerosols impact climate?

Most are shiny, so the reflect and scatter incoming solar radiation. This cools the planet’s surface.

Some, like sulfuric acid, are also greenhouse gases so can warm the surrounding atmosphere.

Unit VIII. Global WarmingC. Causes of Global Warming

6. Aerosols and cloudsHow do aerosols impact climate?• reflect and scatter = cools surface• If also GHG = local atmospheric warming • Particulates increase cloud cover.• Soot (black carbon): cools surface, warms troposphere, changes rainfall patterns.

6. Aerosols and cloudsSoot: Black carbon

Unlike other aerosols, soot is not shiny. It has a low albedo, so absorbs solar radiation. Warms surrounding atmosphere.

It is also opaque, so shades the ground below.

Soot mostly in the troposphere: Earth surface cools, but troposphere warms. Creates instability; rising air and condensation nuclei increase precipitation.

C. Causes of Global Warming6. Aerosols and cloudsChanging cloud cover

High, thin = warming(relatively thin, so allow solar to pass, but capture much of Earth’s LW radiation.)

Low, thick = cooling(Thicker, so higher albedo reflects most solar; even more efficient at trapping Earth’s LW radiation, but little emitted.)

Unit VIII. Global WarmingA. 20th Century WarmingB. A perspective on 20th century warming. C. Causes of Global Warming

• Greenhouse gases +• Solar variability +/-• Volcanism -• Aerosols - (mostly)• Clouds +/-

Unit VIII. Global WarmingA. 20th Century WarmingB. A perspective on 20th century warming. C. Causes of Global WarmingD. Explaining Global WarmingClimate models….. Run with year-to-year changes in forcings.

1. The last century…..

Unit VIII. Global WarmingD. Explaining Global Warming

Climate models….. Run with year-to-year changes in forcings.

1. The last century…..2. The last 1000 years…..

(Medieval Warm Period/Little Ice Age/20th Century warming)

Dust vail index (volcanos)

MWPLIA

20th

Proxy-based temp. record of the last 1000 years

Red = Observations/ProxiesBlue = Model

4. Climate modeling, when driven by known and reconstructed changes in the dominant climate variables, captures reasonably well first and second order changes in global temperature.

This suggests that most of the observed changes in climate over the past 1000 years are due to a combination of:

GHG AerosolsSolar CloudVolcanism

Extending into 21st Century

C. Ammann, NCAR