What factors are key to measuring climate change?

Temperature is the primary measure of climate and can be measured or reconstructed for the Earth's surface, and sea surface temperature (SST).

Precipitation offers another indicator of relative climate variation and may include humidity or water balance.

Biomass and vegetation patterns may be discerned in a variety of ways and provide evidence of how ecosystems change to adapt to climate change.

Sea Level measurements reflect changes in shoreline and usually relate to the degree of ice coverage in high latitudes and elevations.

Solar Activity can influence climate, primarily through changes in the intensity of solar radiation.

Volcanic Eruptions, like solar radiation, can alter climate due to the aerosols that are emitted into the atmosphere and alter climate patterns.

Chemical composition of air or water can be measured by tracking levels of greenhouse gases such as carbon dioxide and methane, and measuring ratios of oxygen isotopes. Research indicates a strong correlation between the percent of carbon dioxide in the atmosphere and the Earth's mean temperature.

What is climate forcing and how does climate forcing relate to climatic variability and change?

Climate forcing refers to specific phenomena that directly influence changes in climatic dynamics. Forces that influence climatic change can be broken down into those beyond the Earth's environmental system (extraterrestrial) and those that relate to internal forces (terrestrial).

Extraterrestrial
Solar variability (sunspots 11, 22 yrs)
Obliquity (tilt on axis~41,000 yrs change)
Precession (wobble on axis due to gravity of sun and moon in 23,000 yr. cycle)
Eccentricity (96,000 yr. change)

Terrestrial
Continental Processes (10⁵ yrs. on up)
Mass and composition of atmosphere
Volcanic activity and strato aerosols
Thermohaline circulation
Cryosphere
Human influences such as land use and combustion of fossil fuels

What is time and how is it measured? What is "powers of ten" temporal scaling?

Humans use thought to conjure up and measure time.

Nature presents two basic measuring units to which many plants and animals intuitively respond:
1. Days- the diurnal cycle, one rotation of the Earth on its axis producing night and day and
2. Years- the annual cycle, one revolution of Earth around the sun.

Years form the fundamental measure of periods of time longer than a day. Using the exponential powers of ten-- otherwise known as "log-ten" or "powers of ten exponents"-- it doesn't take long to go from decades (10) and centuries (10² or 10X10) to billions (10⁹ ) and tens of billions (10¹⁰) of years.

Earth is only an estimated 4.6 X10⁹ years old, and thus there were no annual cycles actually occurring prior to that. Nevertheless, the unit of a year is used to estimate the length of time prior to the existence of Earth, and experts say that period of time between the Big Bang and Earth's first day and year is around 10 billion years. ( See Resources Beyond for more on early and pre-Earth time periods.)

Time can be used to measure distance, as in the term "light year" which is the distance that time travels in a year (9.5 X 10¹² kilometers.) Astronomers also use the term "parsec" so describe distances: one parsec is equal to 3.3 light years.

Between years and days are transition units that we call months, which are roughly the lunar cycle, though not exactly. The Mayan calendar, later incorporated by the Aztec peoples, was an elaborate combination of solar and lunar calendars as well as other astronomical cycles. According to some experts in the Mayan system, the current cycle of time ends December 23, 2012 according to the Mayan calendar, and a new cycle will begin thereafter.

From the unit of a day (which is actually 0.0027 of a year,) smaller units are developed with each day being divided into twenty four hours, every hour (roughly 10^-4 year) subdivided into sixty minutes of sixty seconds in length. Some scientists use seconds as their primary measure and calculate the age of the universe as being a mere 10¹⁹ seconds old. The Swiss watch maker Swatch has recently divided the day up into 1000 beats that they are promoting as a universal time.

Days have obvious parameters that can easily be observed, and lunar and annual cycles have physical markers that can be tracked or can be intuited by animals and plants, but all other measures of time (hours, weeks, seconds, light years, etc.) are abstractions with no basis in nature other than being convenient frameworks to organize time invented by human beings.

Visit the National Institute of Science and Technology's "A Revolution In Timekeeping"and A Walk Through Time website. Another resource is The Origins and Implications of Time from Sanford University. Also see the Why Files' Time Timeline on the development of clocks and other timepieces.