Many past ice ages were caused by… 1.Volcanic activity 2.Photosynthesis 3.Prehistoric humans 4.Changes in the earth’s orbit 5.Sun spots

Climate is “average weather” – Long-term changes in temperature, precipitation, etc. So climate change is changes in “average weather”… …and anthropogenic climate change is changes in “average weather” due to human activity. Anthropogenic climate change

The Intergovernmental Panel on Climate Change (IPCC) is a group of scientists and governments created in 1988 by the World Meteorological Organization (WMO) and the U.N. Environment Programme (UNEP). Mandate: “to provide the decision-makers and others interested in climate change with an objective source of information...” “The IPCC does not conduct any research…” The IPCC

Energy balance Think of the Earth like a water heater: Energy comes into it (E in, e.g., from electric coils) and energy leaves (E out, e.g., heat loss). In steady state, energy in equals energy out (E in = E out ) and so temperature T is constant. Wrap a blanket around the tank and E out goes down. Q: What are the two options for what happens next? A: Either you can keep the same T by reducing E in (conservation!) or T rises to a new steady state.

Source: IPCC 2007, Fig 6.3 Historic climate (Thousands of years)

Historic climate Do we have a theory about what caused historic climate changes? Yes: The theory is that they were driven by variations in the Earth’s orbit (the 120,000- year “Milankovitch cycles”) plus feedbacks.

Source: IPCC 2007 Historic climate

Do we have a theory about what caused historic climate changes? Yes: The theory is that they were driven by variations in the Earth’s orbit (the 120,000- year “Milankovitch cycles”) plus feedbacks. “The next large [change], similar to those that started past Ice Ages, is due to begin in 30,000 years.” [IPCC 2007, FAQ 6.1]

Source: Recent climate

3.6°F 2.7°F 1.8°F 0.9°F cooler warmer Temperature trends (°F per century) since 1920 PNW warmed +1.5 F during the 20 th century Source: UW Climate Impacts Group

Puget Sound Area Air Temperature Source: UW Climate Impacts Group

Source: Recent climate

Source: IPCC 2007, Fig SPM.1 Recent climate

Source: IPCC 2007, Fig 6.3 Recent climate and historic climate (Thousands of years)

The #1 cause of rising GHG levels is… 1.CO2 from deforestation 2.CFCs from industrial activity 3.CO2 from burning fossil fuels 4.Methane from cows

The #2 cause of rising GHG levels is… 1.CO2 from deforestation 2.CFCs from industrial activity 3.CO2 from burning fossil fuels 4.Methane from cows

Recent climate Do we have a theory about what caused recent climate changes? Yes: The theory is that they are driven by anthropogenic activity (burning fossil fuels, deforestation, etc.) plus feedbacks. IPCC 2007: “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

Arrhenius (Swedish chemist), 1896

Greenhouse gases Most incoming energy is at wavelengths in the visible spectrum. Most outgoing energy is at longer infrared wavelengths. Greenhouse gases (water vapor, CO 2, …) interact with outgoing (infrared) but not incoming (visible).

Anthropogenic climate change / global warming Like an extra blanket on the water heater, increased levels of CO 2 and other greenhouse gases reduce E out (energy out). Since E in (energy in) is not changing much, we have E in > E out and so the Earth is out of energy balance and so T rises. Q: What happens to temperatures in the stratosphere (up 10-50km / 6-31miles)?

What happens to temperatures in the stratosphere? 1.Rise 2.Fall

Stratospheric temperature Q: What happens to temperatures in the stratosphere (up 10-50km / 6-31miles)? A: Stratospheric temps are falling (IPCC 2007). Q: How sure are scientists about anthropogenic climate change?

1990 (1 st Assessment Report) “[The observed warming in the 20th century] is broadly consistent with predictions of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability…”

1995 (2 nd Assessment Report) “The balance of evidence suggests a discernable human influence on global climate.”

2001 (3 rd Assessment Report) “There is new and stronger evidence… most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.”

2007 (4 th Assessment Report) “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

Recent climate IPCC 2007: “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” Q: Are there other (natural) forces at work? A: Yes, e.g., E in from the sun has been going up. But this is about 1/10 th of anthropogenic impacts. Q: Can other theories explain recent climate? A: No.

Source: IPCC 2007, FAQ 9.2, Fig 1

Future climate Two main uncertainties about future climate. Uncertainty #1: What’s going to happen with emissions of CO 2 and other GHGs? Uncertainty #2: What’s going to happen with positive or negative feedback loops? Positive feedback loops amplify. Ex: as T rises, less snow/ice, lower albedo means more sunlight is absorbed rather than reflected. Negative feedback loops tone down. Ex: As T rises, more water vapor, so more clouds, and clouds reflect incoming sunlight. (Note: Clouds are key!)

Climate sensitivity: How much would T go up in a new steady state with doubled CO 2 ? Source: UW atmospheric physics professor Marcia Baker

Future GHG emissions: Going up for now… Source: IPCC 2007, Fig. SPM.3

Put Uncertainties #1 and #2 together… Source: IPCC 2007, Fig. SPM.5. Warming is relative to

T/F: Scientists know that Katrina resulted from climate change 1.True 2.False

T/F: Climate change could lead to an ice age by stopping the MOC 1.True 2.False

Future climate Global temperature has been rising ≈0.2°C (≈0.5°F) per decade, and IPCC 2007 predicts similar increases “for the next two decades.” Even if we stopped cold turkey, some 21 st century warming (≈0.6°C, ≈1.1°F) is projected because we are not in energy balance. [IPCC 2007] Atmospheric CO 2 concentrations (280ppm pre- industrial, 379ppm in 2005) are going up ≈2ppm per year. [IPCC 2007 WG1SPM, p. 2]

Source: Recent climate