Global Warming: the history Why should we be worried about overall global climate change?

What is global warming?  Before we define global warming, we need to remember a few things about the atmosphere and climate.  The atmosphere is defined as a thin layer of gases that surrounds our Earth. Without the atmosphere, organisms that survive on Oxygen would not be able to survive. This atmosphere also helps to distribute the heat gained from the sun’s rays.  The atmosphere consists of  78% N2  21% O2  1% Ar  And other trace amounts of permanent gases

Our Climate’s History  Since the time of Earth’s existence, the atmosphere has changed. Its composition began primarily as hydrogen and helium. As Earth processes took place – such as volcanic eruptions, the atmosphere began to change. It is hypothesized that Earth’s atmosphere was very similar to Venus’ atmosphere – primarily CO2, which made the Earth an unbearable place to live.

First Atmosphere  Composition - Probably H 2, He  These gases are relatively rare on Earth compared to other places in the universe and were probably lost to space early in Earth's history because  Earth's gravity is not strong enough to hold lighter gases  Earth still did not have a differentiated core (solid inner/liquid outer core) which creates Earth's magnetic field which deflects solar winds.  Once the core differentiated the heavier gases could be retained

Second Atmosphere  Produced by volcanic out gassing. Gases produced were probably similar to those created by modern volcanoes (H 2 O, CO 2, SO 2, CO, S 2, Cl 2, N 2, H 2 ) and NH 3 (ammonia) and CH 4 (methane)  No free O 2 at this time (not found in volcanic gases).  Ocean Formation - As the Earth cooled, H 2 O produced by out gassing could exist as liquid in the Early Archean, allowing oceans to form.  Evidence - pillow basalts, deep marine beds in greenstone belts.

Addition of O 2 to the Atmosphere  Today, the atmosphere is ~21% free oxygen.  Oxygen Production  Photochemical dissociation - breakup of water molecules by ultraviolet  Produced O 2 levels approx. 1-2% current levels  At these levels O 3 (Ozone) can form to shield Earth surface from UV  Photosynthesis - CO 2 + H 2 O + sunlight = organic compounds + O 2 - produced by cyanobacteria, and eventually higher plants - supplied the rest of O 2 to atmosphere. Thus plant populations  Oxygen Consumers  Chemical Weathering - through oxidation of surface materials (early consumer)  Animal Respiration (much later)  Burning of Fossil Fuels (much, much later)  During the Proterozoic the amount of free O 2 in the atmosphere rose from %. Most of this was released by cyanobacteria, which increase in abundance in the fossil record 2.3 Ga. Present levels of O 2 were probably not achieved until ~400 Ma.

 Life on Earth is supported by the atmosphere, solar energy, and our planet's magnetic fields. The atmosphere absorbs the energy from the Sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects us from high-energy radiation and the frigid vacuum of space. Our Atmosphere

The Troposphere  From the Earth’s surface through the bottom layer of atmosphere, called the troposphere, temperature decreases with altitude. Weather occurs in this layer. It’s also the layer we live in.

Stratosphere  The next layer up is called the stratosphere. In the stratosphere, temperature increases with altitude. This is because of ozone. When the ozone in this layer absorbs UV light from the sun, it increases in temperature.

Mesosphere  In the mesosphere, ozone concentration decreases. This means there is less absorption of UV light in this layer of atmosphere. Do you think the temperature would increase or decrease In this layer of the atmosphere?

Thermosphere  In the upper atmosphere, called the thermosphere, temperatures are HOT. This is because incoming energy from the sun heats the molecules up. Well, this occurs because short-wave, high-energy solar radiation is absorbed by the (relatively few) molecules of oxygen and nitrogen.

Earth Naturally goes through a warming and cooling periods….  Serbian astronomer Milutin Milankovitch, who calculated the slow changes in the earth's orbit by careful measurements of the position of the stars, and through equations using the gravitational pull of other planets and stars.  He determined that the earth "wobbles" in its orbit. The earth's "tilt" is what causes seasons, and changes in the tilt of the earth change the strength of the seasons. The seasons can also be accentuated or modified by the eccentricity (degree of roundness) of the orbital path around the sun, and the precession effect, the position of the solstices in the annual orbit.  The Milankovitch or astronomical theory of climate change is an explanation for changes in the seasons which result from changes in the earth's orbit around the sun.

What does this mean?  The combination of the 41,000 year tilt cycle and the 22,000 year precession cycles, plus the smaller eccentricity signal, affect the relative severity of summer and winter, and are thought to control the growth and retreat of ice sheets.  Cool summers in the northern hemisphere, where most of the earth's land mass is located, appear to allow snow and ice to persist to the next winter, allowing the development of large ice sheets over hundreds to thousands of years.

Earth’s Natural Greenhouse Effect  Greenhouse gases like water vapor, carbon dioxide, methane and nitrous oxide trap the infrared radiation released by the Earth's surface.  The atmosphere acts like the glass in a greenhouse, allowing much of the shortwave solar radiation to travel through unimpeded, but trapping a lot of the longwave heat energy trying to escape back to space. This process makes the temperature rise in the atmosphere just as it does in the greenhouse. This is the Earth's natural greenhouse effect and keeps the Earth 33°C warmer than it would be without an atmosphere, at an average 15°C.

Warming the Earth Naturally…  The Sun, which is the Earth's only external form of heat, emits solar radiation mainly in the form of shortwave visible and ultraviolet (UV) energy. As this radiation travels toward the Earth, 25% of it is absorbed by the atmosphere and 25% is reflected by the clouds back into space. The remaining radiation travels unimpeded to the Earth and heats its surface. The Earth releases a lot of energy it has received from the Sun back to space. However, the Earth is much cooler than the Sun, so the energy re-emitted from the Earth's surface is much weaker, in the form of invisible longwave infrared (IR) radiation, sometimes called heat.