· Are human activities responsible for the warming? Separating out the impact of human activity from natural climate variation is extremely difficult. Nonetheless, the IPCC concluded there is a 'discernible human influence' on climate. This means the observed global warming is unlikely to be the result of natural variability alone and that human activities are at least partially responsible.

· What Role does Solar Radiation play in Determining the Earth's Climate? Energy or solar radiation enters the atmosphere as sunshine. The sky reflects a portion of this radiation, the rest is absorbed by the Earth and is eventually released again as thermal radiation. Both man-made and naturally occurring events can limit the amount of solar radiation at the earth's surface. Urban air pollution, smoke from forest fires, and airborne ash resulting from volcanic activity reduce the solar resource by increasing the scattering and absorption of solar radiation. Some scientists think of the radiation budget in terms of a balance. If the Earth gets more energy from the Sun, the Earth heats up and emits more thermal energy. This brings the radiation budget into balance. If the Earth emits more of this thermal energy than it absorbs from solar radiation, the Earth cools off. As it cools off, the Earth emits less energy. This change also brings the radiation budget back into balance.

Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well- known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events

· How are the amounts of greenhouse gases measured? Non-isotopic measurements of greenhouse gases are generally made with two techniques: Non-Dispersive Infrared (NDIR) analysis, or by gas chromatography. The amount of CO2 in the atmosphere is large enough that the NDIR method of measurement works well. An NDIR analyzer relies on the same principle of IR absorption that makes greenhouse gases important in the first place. An infrared analyzer consists of an infrared source at one end, and an infrared detector separated by a gas cell. The gas of interest is passed through this cell, and absorbs some of the infrared radiation coming from the source. The detector converts the amount of IR reaching it to a usable signal, such as a voltage. So as the concentration of CO2 changes in the sample, the signal from the detector changes. By flowing a gas with a known amount of CO2 through the cell, we can calibrate the analyzer so that the voltage output from the detector can be converted into amounts of CO2.

· Where are these gases measured? GMD's Carbon Cycle group (CCGG) makes continuous and discrete measurements of greenhouse gases at numerous worldwide surface sites, towers, aircraft, and ships of opportunity. There is a world map of the many different sampling sites. Air samples are collected in glass flasks from sampling sites of the NOAA GMD CCGG Cooperative Air Sampling Network and returned to the CCGG laboratory in Boulder, Colorado for analysis. In-Situ, continuous measurements are made at the 4 baseline GMD Observatories, and at 2 tall tower sites in the United States

· How much CO 2 is in the atmosphere? For the year 2006, the estimated global average CO 2 amount was about 381 parts per million (ppm). If we use a value of 5.13 ×10 18 kg for the mass of the dry atmosphere (excluding water vapor), and take into account that the average molecular weight of air equals 29.0 g/mole and the atomic weight of carbon 12.0 g/mole, then 1 ppm of CO 2 corresponds to 2.12 Gt (billion metric ton) of carbon. This gives us a value of about 808 Gton of carbon in the atmosphere. If we include the mass of the two oxygen atoms in each CO 2 molecule, the total mass of CO 2 would be 2960 Gton. Watch out: in the news and in reports you may see emissions expressed either as tons of carbon or as tons of carbon dioxide. The latter is 3.67 times heavier than the former, although the same number of CO 2 molecules is involved