This chart is from data supplied by the United States National Climate Data Center out of Asheville, North Carolina. It is composed from monthly data from land and ocean temperature measurements. The sharp spikes in the blue line correspond to El Nino (warm) and La Nino (cold) oscillations that periodically occur in the Pacific Ocean. El Nino/La Nina areas are just part of the Pacific Ocean, so it is rather surprising that they can shift global temperatures as much as they do. However, as can be seen on the chart, the most dramatic shifts last only a few months.
The red line is a rolling 12 month average to help smooth out the spikes. Notice, how even after averaging the data for 12 months there are still obvious oscillations that last about 4 years. This illustrates that besides the dramatic spikes, there are also longer term El Nino/La Nina trends that cycle about every 4 years.
To smooth out these longer term cycles, a 5 year running average is constructed in the green line. It smoothes out the El Nino/ La Nina oscillations fairly well. Coincidentally, the 5 year running average for 1979 was nearly equal to the average for the century.
In 1991, a large volcanic erruption occured at Mt Pinatubo. This cooled the earth for a few years and the cooling trend is almost noticeable, but complicated by the El Nino/ La Nina osciallaitons. Never the less, the long term warming trend over the last 30 years is clearly visible with only minor shifts.