What are negative forcings?

Figure 1.  Estimated change of climate forcings between 1850 and 2000, based on with five principal aerosols delineated.
Source: http://www.giss.nasa.gov/research/features/200105_senate/page5.html

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Negative forcing tends to cool the earth's surface.

While aerosols can have either positive or negative contributions to radiative forcing, the net effect of all aerosols added to the atmosphere has likely been negative. The best estimate of aerosols’ direct cooling effect is -0.5 Watts per square meter; the best estimate for their indirect cooling effect (by increasing the reflectivity of clouds) is -0.7 Watts per square meter, with an uncertainty range of -1.8 to -0.3 Watts per square meter. Therefore, the net effect of changes in aerosol radiative forcing has likely resulted in a small to relatively large cooling effect.

Figure 2. Climate forcings in the past 50 years, relative to 1950, due to six mechanisms. The first five forcings are based mainly on observations, with stratospheric H2O including only the source due to CH4 oxidation. GHGs include the wellmixed greenhouse gases, but not O3 and H2O. The tropospheric aerosol forcing is uncertain in both its magnitude and time dependence.

The above figure shows stratospheric aerosols which tend to be a negative forcing.

Source: http://www.giss.nasa.gov/research/features/200105_senate/page5.html