So since 1975 we've warmed up quite a bit.
The figure above shows the monthly temperature deviations from a seasonally adjusted average for the lower stratosphere - Earth's atmosphere from 14 to 22 km (9 to 14 miles). Red is an increase in the temperature from the average, and blue is a decrease in temperature. The large increase in 1982 was caused by the volcanic eruption of El Chichon, and the increase in 1991 was caused by the eruption of Mt. Pinatubo in the Philippines. November 2000 was the coldest month on record for stratospheric temperatures.
This chart shows the monthly temperature changes for the lower troposphere - Earth's atmosphere from the surface to 8 km, or 5 miles up. The temperature in this region is more strongly influenced by oceanic activity, particularly the "El Niño" and "La Niña" phenomena, which originate as changes in oceanic and atmospheric circulations in the tropical Pacific Ocean. The overall trend in the tropospheric data is near zero, being +0.04 C/decade through Feb 2002. Click on the charts to get the numerical data.
Why, you ask? Well, it's been asked before.
The stratosphere warms and cools radiatively. It cools by radiating heat into space. It warms by receiving longwave radiation from the Earth's surface. If that radiation is trapped near the surface (which is what the greenhouse effect in the atmosphere does) then the radiation doesn't reach the stratosphere and the stratosphere cools. So if an increasing amount of LW radiation is being trapped, there should be a cooling signal in the stratosphere.
And there is. Voila.