Earth’s Climate System Climate – long term atmospheric conditions in a region Earth’s climate includes interactions of: Atmosphere Hydrosphere Geosphere Biosphere Cryosphere Climate system – exchanges of energy and moisture between these spheres

Earth’s Climate System

Earth’s Climate System Feedback loops – modify atmospheric processes Positive feedback loops – enhance initial change Negative feedback loops – counteract initial change

Determining Causes of Earth’s Climate Change Paleoclimatology Proxy data – indirect evidence using natural recorders of climate variability Sea floor sediments Coral deposits Glacial ice rings Tree rings Pollen Historical documents

Temperature: the last 400,000 years From the Vostok ice core (Antarctica) That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear. The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

Natural Climate Change External Forcing: Internal Forcing: The agent of change is outside of the Earth-atmosphere system The agent of change is within the Earth-atmosphere system itself

External Forcing Variations in solar output Orbital variations Meteors

Natural Causes of Climate Change Solar energy changes Variable energy from the Sun over time Luminosity Sunspots

Solar Variations Sunspots correlate with solar activity More sunspots, more solar energy Sunspots are the most familiar type of solar activity.

SOLAR ACTIVITY Sunspots are the most familiar type of solar activity.

THE SOLAR CYCLE Sunspot numbers increase and decrease over an 11-year cycle Observed for centuries. Individual spots last from a few hours to months. Studies show the Sun is in fact about 0.1% brighter when solar activity is high.

The Seasons The Earth has seasons because the axis is tilted. Earth rotates on its axis as it orbits the Sun. Earth’s axis ALWAYS points in the same direction… but Earth’s location in its orbit around the Sun determines whether the NH or SH is pointed toward the Sun.

The Seasons NH Summer Solstice NH Autumnal Equinox NH Winter Solstice June 20-21 NH Autumnal Equinox September 22-23 NH Winter Solstice December 21-22 NH Vernal Equinox March 20-21

Earth’s orbit: an ellipse Perihelion: place in the orbit closest to the Sun Aphelion: place in the orbit farthest from the Sun

Earth’s orbit: an ellipse Perihelion: place in the orbit closest to the Sun Aphelion: place in the orbit farthest from the Sun

THE MAUNDER MINIMUM An absence of sunspots was well observed from 1645 to 1715. The so-called “Maunder minimum” coincided with a cool climatic period in Europe and North America: “Little Ice Age” The Maunder Minimum was not unique. Increased medieval activity correlated with climate change.

Natural Causes of Climate Change Variations in Earth’s Orbit Milankovitch Theories Eccentricity of Earth’s orbit Obliquity of Earth’s axis Precession of Earth’s axis

Orbital changes Milankovitch theory: Serbian astrophysicist in 1920’s who studied effects of solar radiation on the irregularity of ice ages Variations in the Earth’s orbit Changes in shape of the earth’s orbit around sun: Eccentricity (100,000 years) Wobbling of the earth’s axis of rotation: Precession (22,000 years) Changes in the tilt of earth’s axis: Obliquity (41,000 years)

Eccentricity: period ~ 100,000 years

Natural Causes of Climate Change Volcanic eruptions Volcanic ejecta may block sunlight Need many eruptions in short time period Not observed in recent history

Natural Causes of Climate Change Movement of Earth’s Plates Change ocean circulation Extremely slow process Climate change would be very gradual over millions of years Linked to Pleistocene Ice Age, Little Ice Age, Medieval Warm Period

Documenting Human-Caused Climate Change Intergovernmental Panel on Climate Change (IPCC) Global group of scientists Published assessments since 1990 Predict global temperature changes of 1.4–5.8°C (2.5–10.4°F) Climate change models can mimic modern conditions only if human emissions are taken into account.

Atmosphere’s Greenhouse Effect Global warming – increase in Earth’s global temperatures Greenhouse effect – keeps Earth’s surface habitable Incoming heat energy is shorter wavelengths Longer wavelengths – some trapped, some escape, net warming effect

Earth’s Heat Budget Addition to or subtraction from heat on Earth Incoming radiation from Sun shorter wavelengths Outgoing radiation from Earth longer wavelengths Rates of energy absorption and reradiation must be equal

Earth’s Heat Budget

Greenhouse Gases Water vapor Carbon dioxide Most important 66–85% of greenhouse effect Carbon dioxide Natural part of atmosphere Greatest relative contribution from human activities Burning of fossil fuels

Atmospheric Carbon Dioxide

Greenhouse Gases Methane Other trace gases Second most abundant human-caused greenhouse gas Great warming power per molecule Landfill decomposition Cattle Other trace gases Nitrous oxide, CFCs, ozone

Human-Caused Greenhouse Gases