Read here. Climate. Cycles. Change. Put the three together and voilà, you get 'C3'. We've always been influenced by the idea that natural climate change is constantly happening, which causes oscillating temperature cycles from cold to warm back to cold, and etc.
Now, there is more confirming evidence as an analysis was done on thermometer temperatures going back to 1880 on widely separated geographical regions. What did the analysis find? It would seem a natural sine wave - a cycle that switches from cold-to-warm-back-to-cold-etc. exists. Isn't nature just amazing?
"Taking these ten locations from across the globe andsuperimposing the anomaly data produced a sine wave-like pattern with distinct cooling from the early 1940s to mid-1970s followed by warming to present; for many of the locations the older data was warmer, or at least as warm as present. Now I had seen this before with many individual stations, but it really impressed me to see the pattern matching from such far-flung locations.....So is this “sine wave” the true climate signal? It would seem so, although we can’t expect it always to be so regular. Choosing stations that are more closely geographically located does give a more homogeneous shape to the wave.....For us the take-home message of this study was simply how widespread and consistent the wave pattern is, and this, ultimately is very convincing of the veracity of the arguments against CO2 as a primary cause of current warming. From the physics I don’t doubt it has a role in warming, but its role needs to be disentangled from the large magnitude natural climate swings that are clearly present all over the world..."