As can be seen from the chart, atmospheric CO2 levels have been steadily rising for many years. It is believed that rising CO2 levels are primarily responsible for warmer average global temperatures. If this is true, then there are statistical tests that can be applied. It is not necessary to design a complex super computer model to determine the validity of the CO2 warming hypothesis.
The most common statistical test is to construct a control limit of several standard deviations below the mean trend line. If a global temperature anomaly falls more than 2.5 standard deviations below the long term trend, then such an event would have less than 1% likelihood or about once every 100 years in a normal distribution of expected warming from CO2 levels rising as they have been.
The last 30 years of data show a yearly standard deviation of 0.07 degree celsius from the trend line. So if there were about a 0.17 degree celsius drop below the trend, for a single year average, then this should trigger a search for a “another cause”. Each of the last 30 years has been above the 2.5 standard deviation low line. This includes the years following the eruption of Mount Pinatubo in June 1991, the most climatically significant volcanic eruption of the period.
Another statistical test often used, is a 7 point sequence of steadily rising (or falling) data. This apparently has about the same likelihood as a three sigma event, in a normal distribution. So, if we see seven years in a row, where the temperature anomaly is falling lower and lower each and every year, while greenhouse gases concentrations are rising at their current rate, this would also trigger the search for a special cause. So far, the greatest number of consecutive cooling years is 3, which is this year. Maybe we will see 4 more, but I seriously doubt it.
Skeptics have been searching for an explantion of the warming that does not include human causes. So far, there are no other credible causes. Of course, if a known forcing comes from an unpredictable event, such as a volcanic eruption, then the forecast output would have to be adjusted accordingly for the known forcing, before comparing with the actual data.
Here is a historical list of several climatically significant volcanic eruptions that caused global cooling in the past.
Kuawe (1452-1453) -- An underwater vulcano in the South Pacific. In Sweden, grain tithes fell to zero as crops failed; western U.S. bristlecone pines show frost damage; and the growth of European and Chinese trees was stunted in 1453–57. According to the history of the Ming Dynasty in China in the spring of 1453, "Nonstop snow damaged wheat crops." Later that year, as the dust obscured the sunlight, "Several feet of snow fell in six provinces; tens of thousands of people froze to death. "Early in 1454, "it snowed for 40 days south of the Yangtze River and countless died of cold and famine." Lakes and rivers were frozen, and the Yellow Sea was icebound out to 20 km from shore.
HUAYNAPUTINA (1600) -- A stratovolucano location in Peru. The explosion had effects on climate around the Northern Hemisphere, where 1601 was the coldest year in six centuries, leading to a famine in Russia that eventually lead to an estimated 2 million deaths. From 1600 to 1602, Switzerland, Latvia and Estonia had exceptionally cold winters. The wine harvest was late in 1601 in France, and in Peru and Germany wine production collapsed. Peach trees bloomed late in China, and Lake Suwa in Japan froze early. Sulfuric acid levels deposited in the Greenland ice cap are larger than that from Krakatau (1883).
LAKI (1783) -- The eastern U.S. recorded the lowest-ever winter average temperature in 1783-84, about 4.8 degree C below the 225-year average. Europe also experienced an abnormally severe winter. Benjamin Franklin suggested that these cold conditions resulted from the blocking out of sunlight by dust and gases created by the Iceland Laki eruption in 1783. The Laki eruption was the largest outpouring of basalt lava in historic times. Franklin's hypothesis is consistent with modern scientific theory, which suggests that large volumes of SO2 are the main culprit in haze-effect global cooling.
TAMBORA (1815) -- Thirtythree years later, in 1815, the eruption of Mt. Tambora, Indonesia, resulted in an extremely cold spring and summer in 1816, which became known as the year without a summer. The Tambora eruption is believed to be the largest of the last ten thousand years. New England and Europe were hit exceptionally hard. Snowfalls and frost occurred in June, July and August and all but the hardiest grains were destroyed. Destruction of the corn crop forced farmers to slaughter their animals. Soup kitchens were opened to feed the hungry. Sea ice migrated across Atlantic shipping lanes, and alpine glaciers advanced down mountain slopes to exceptionally low elevations.
KRAKATAU (1883) -- Eruption of the Indonesian volcano Krakatau in August 1883 generated twenty times the volume of tephra released by the 1980 eruption of Mt. St. Helens. Krakatau was the second largest eruption in history, dwarfed only by the eruption of neighboring Tambora in 1815 (see above). After the Krakatau eruption, average global temperatures fell by as much as 1.2 degrees Celsius. Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888. Brilliant sunsets and prolonged twilights were due to the spread of aerosols throughout the stratosphere.