A question about trends and noisy data

DnD Addict

Not sure if this belongs here or in the math forum.

Anyways, I have read that the signal of AGW is about 0.018C/yr, while the random noise from solar variance, el nino/la nina, etc. are about 0.2C. I have also heard claims that the trend in global temperature has leveled off or even claims that it is now negative.

Is the timeframe from 1998 to the present long enough to make a valid claim about what the trend is doing, or is it impossible to seperate out the signal from the noise in that timeframe?

If it's not long enough, how long do you need given said SNR to make a meaningful claim about the trend?

sylas

This is the right forum, because the answer for how long depends on the particular phenomenon itself. The usual view with respect to global temperature is that trend shows up over around 30 years or so. Shorter periods will tend to show the trend also, but with less confidence. Over 10 years, you are likely to have too strong an influence from short term variation to be confident about trend. That applies, of course, just as much for a decade where temperatures show strong trend as for a decade where trend is weak to non-existent.

From the World Meteorological Organization FAQ:

What is Climate?

Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.

Cheers -- sylas