There is clear evidence of a cooling effect in the GHCN-Daily data resulting from large volcanic eruptions during the 19th and 18th century. If an eruption of a similar size to Tambora were to occur today it would (temporarily) cancel out all CO2 warming.
Note that early ‘Global’ temperatures are dominated by European stations. This is probably why Laki appears to be as strong as Tambora, which had a much larger effect in Asia.
Laki, Iceland – 1793
Globally, those 95 Mt of sulphuric dioxide reacted with atmospheric water to form 200 Mt of sulphuric acid aerosols. Almost 90% of that sulphuric acid was removed in the form of acid rain or fogs, while 10% stayed aloft for over a year. This might explain why northern hemisphere temperatures were 1.3ºC below normal for 2-3 years after the eruption. (Wired)
Tambora, Indonesia – 1815
Many volcanologists regard the Mount Tambora eruption as the largest and most-destructive volcanic event in recorded history, expelling as much as 150 cubic km (roughly 36 cubic miles) of ash, pumice and other rock, and aerosols—including an estimated 60 megatons of sulphur—into the atmosphere. As that material mixed with atmospheric gases, it prevented substantial amounts of sunlight from reaching Earth’s surface, eventually reducing the average global temperature by as much as 3 °C (5.4 °F). (Britanica)
Consiguina, Nicaragua, 1835
The January 1835 eruption of Cosigüina volcano, Nicaragua, ranks among the Americas’ largest and most explosive historical eruptions, but whether it had effects on global climate remains ambiguous. New petrologic analyses of the Cosigüina deposits reveal that the eruption released enough suphur to explain a prominent circa A.D. 1835 sulphate anomaly in ice cores from both the Arctic and Antarctic. A compilation of temperature?sensitive tree ring chronologies indicates appreciable cooling of the Earth’s surface in response to the eruption, consistent with instrumental temperature records. We conclude that this eruption represents one of the most important sulphur?producing events of the last few centuries and had a sizable climate impact rivaling that of the 1991 eruption of Mount Pinatubo. (Longpré et al.)
Krakatoa, Indonesia, 1883
In May 1883, the captain of the Elizabeth, a German warship, reported seeing clouds of ash above Krakatau. He estimated them to be more than 6 miles (9.6 km) high. For the next two months, commercial vessels and chartered sightseeing boats frequented the strait and reported thundering noises and incandescent clouds….On the morning of the 27th August, four tremendous explosions, heard as far away as Perth, Australia, some 2,800 miles (4,500 km) distant…… The explosions hurled an estimated 11 cubic miles (45 cubic km) of debris into the atmosphere, darkening skies up to 275 miles (442 km) from the Volcano…Within 13 days, a layer of sulphur dioxide and other gases began to filter the amount of sunlight able to reach Earth. The atmospheric effects made for spectacular sunsets all over Europe and the United States. Average global temperatures were as much as 1.2 degrees cooler for the next five years. (LiveScience)
By comparison the 20th century and the 21st century so far has seen far less Volcanic activity. Mount Pinatubu caused a temporary short term drop in global temperatures of about 0.4C for 2 years. Another Tambura type event would be far more serious with longer lasting effects. After-all we know how cold it can get at night!