Because it's a new year, I calculated the exact averages, taking the right number of days in each month (including leap years) into account. The ranking is as follows:
- 1998: 0.549 °C
- 2010: 0.510 °C
- 2005: 0.374 °C
- 2003: 0.358 °C
- 2002: 0.334 °C
It was the 12th year in a row that wasn't able to surpass 1998 so the lukewarm year 1998 remains the "hottest one", or the "least cold year", depending on your preferences, on the RSS AMSU record.
El Nino and La Nina
Can these numbers be understood by looking at the ENSO index, i.e. the El Nino and La Nina episodes? They partially can.
While the data from 1979 show 0.15 °C per decade of warming, none of it can be observed, in a statistically significant way, from 1998.
If you want to use one three-month period of ENSO data to predict the annual average global temperature for a year, it's optimal to take the three-month period centered on January of the same year (the beginning of the year) - the Dec-Jan-Feb period. This one has the highest correlation with the temperature of the whole year - a fact that is compatible with an approximate 6-month delay in the impact of ENSO because the average of year is most tightly correlated with the temperature of the middle of the year, i.e. June or July.
If you write down the optimum linear fit for the average temperature of the whole year, you get
RSS anomaly for year = 0.09 °C + 0.07 x ENSO (Jan)where ENSO (Jan) is the Dec-Jan-Feb ONI 3.4 average index (also in °C) from the beginning of the same year.
Because the 2009-2010 El Nino was just somewhat weaker than the 1997-1998 El Nino, the predicted annual mean temperature, according to the formula above, is exactly 0.30 °C cooler than the actual observed RSS annual mean temperature. This fact is both true for 1998 and 2010. So if you adjust the RSS temperatures for the El Nino index, there has been no noticeable extra warming or cooling from 1998.
However, even with the adjustments, there has been a warming by 0.15 °C per decade since 1979 when the satellite measurements began. This fact remains unchanged if you add the ENSO adjustments. However, the statistical data make it viable to say that all of the warming occurred before 1999.
If you want to make predictions for 2011, the relevant 2011 ENSO index - which is right now - is about -1.7 °C, about 3.4 °C cooler than the relevant ONI index for 2010 which was 1.7 °C one year ago. Multiply the difference by the coefficient 0.07 above to get 0.24 °C or so. So 2011 is predicted to be 0.24 °C cooler than 2010, plus some noise.
If this prediction were accurate, the RSS anomaly 0.55 °C from 2010 would drop to 0.31 °C which would mean that 2011 would be tied with 2007 as the 6th warmest year.
