There is a report today from the UN as part of the Copenhagen Conference that claims this decade is the warmest on record and that 2009 was the fifth warmest on record (e.g. see). This claim is based on surface temperature data (from CRU, GISS and NCDC), which use essentially the same raw data and then perform slightly different analyses; see.

Following I have listed below examples of what the peer-reviewed research reports on this surface data:

There are a number of signficant uncertainties and biases in these temperatures when they are used to diagnose the magnitude of global warming; e.g. see Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229.

This includes a clear significant warm bias with respect to quantifying the magnitude of global warming as diagnosed, for example, by the peer reviewed paper Klotzbach, P.J., R.A. Pielke Sr., R.A. Pielke Jr., J.R. Christy, and R.T. McNider, 2009: An alternative explanation for differential temperature trends at the surface and in the lower troposphere. J. Geophys. Res., 114, D21102, doi:10.1029/2009JD011841.

The conclusion from these (and other papers on this subject) is that the claim that the first decade of this century is the warmest on record is based on temperature data sets that clearly overstate the magnitude of global temperature anomaly.

However, the most robust data we have that extends back several decades (to 1979) does confirm that, as represented by lower tropospheric temperatures, the first century of this century are above average in terms of this climate metric. This can be seen in the below of the lower tropospheric temperature diagnosed by RSS data (see Figure 7)

It is, therefore, correct to state that the first decade of the century is the warmest in this record, but it is also correct to state that there is no statistically significant trend since early in this current decade (where the large spike in 1998 is not involved). There has been warming, in terms of the temperature anomalies in the last two years, however, so it is correct to report that lower tropospheric warming is currently occurring.

Both the RSS and MSU data are available at NCDC. The latest available global average lower tropospheric temperature anomaly relative to the 1979-current time period (which is for November 2009) is +0.50C (UAH) with the current linear long-term trend of 0.13C per decade (UAH).