I have temperature data for 29 yrs, using first 20 yrs as baseline period I have calculated anomaly for the rest 9 years, can I use the same baseline value for the first 20 yrs?
Deepthi,
A baseline value is just a convenient, conventional reference to evaluate deviations, and possibly trends. That reference is ideally stable over time (hence the choice of a long period), but there is nothing magic about 20 years. In fact, climatologists used to settle for 30 year averages, but that is fairly arbitrary in the context of climate change anyway.
If you have questions about how long that period should be, you could compute the mean for periods of various lengths and see how much they differ from each other, and evaluate what impact that has on the deviations.
When you analyze your results, just remember that the status of the baseline is different during and after the reference period.
Cheers, Michel.
Dear Deephti
Yes you can reasonable compute your anomalies for the first 20 years. To be really sure pay attention that you first reference 20 years do not include "accident" able to introduce spurious anoamlies, trends, breack points and so on.The best would to test different periods of reference and to evaluate the sensitivity.
As mentioned by Michel, climatologists prefer to use a period of 30 years, i.e. the period 1971-2000, used for instance by the Berkeley center: http://berkeleyearth.lbl.gov/ . In consulting this website you can find some details about the question.
Regards,
G.C.
Dear Deepthi,
Yes you can reasonably use it because this is a reference and are conventionally chosen.
Soumendu Chatterjee
A baseline of 20 years is already quite close to the "climate" standard of 30 years.
When you use this baseline, you can easily see warmer and cooler "than average" periods in your data. When you continu to use the same baseline for the next 9 years, you still will able to analyse whether the new data have periods that are warm or cold compared to your baseline period.
The baseline is what it is: a baseline. It is not "current climate". When climate is changing, the 10-y or 20-y or 30y-averages also will change. Because of decadal slow weather phenomena and oscillations, climatologists prefer to define "climate" as the average over 30y (not 10 or 20y). Current climate should then be the weather of 1985-2014. As changes are small, climate averages are only updated every 10y; in the Netherlands we now use 1981-2010 for our current climate.
It depends on what you are to examine. If you are to analyze the climatology, you’d better use the current reference period as recommended by the WMO, 1981-2010, or all the years available; If you are to analyze climatic change and variability, you can use the first 20 years, or all the years available, as base period for calculating anomalies. Data in base period should be relatively complete in any circumstances.
Yeah, it is quite better to continue with the same baseline to evaluate the changes taking place in those other 9 years.