To say that the planet is warming is equivocal: one can compare the, e.g., the average global temperature for the month of March with the average global temperature for the month of March during the decade 1960 to 1970. Or one could compare it to the averages from the decade 1930 to 1940. Or one could compare it a monthly average for any single arbitrary year.
One can find sets of data points to support the conclusion that the planet is warming, that the planet is cooling, or that the planet is holding steady. The choice of which set of points is taken as a baseline normal is arbitrary. Extrapolations into the future can be made to trend upwards or downward, depending on how far back in time one begins the best-fit line which one extrapolates.
If the planet is cooling or warming, it must be relative to some measured point. Choosing that reference point is stacking the deck. Especially because our data are limited to recent years. Extensive measures prior to, e.g., the year 1800 are not plentiful enough to be statistically significant. Because these claims are global, worldwide data points would be needed to substantiate them. Yet large parts of the world lack any reliable observations prior to the last few decades.
It may not be possible to substantiate whether or not the planet is warming, cooling, or holding even on a long-run basis. We'd need several centuries of reliable measured temperatures from numerous scattered points on all seven continents. We do not have such data.
It is possible to argue that short-term global trends, warming or cooling, are underway; data might be available to verify such claims. But short-run trends are not able to tell us about long-run trends, because the planet constantly seesaws through various short-term temperature fluctuations.
Likewise, localized cooling and warming trends can be substantiated when data are available, but localized trends also are unable to shed light on global trends; it has been documented that one continent may experience a multi-year warming while another experiences a multi-year cooling.
Lacking precise measurements over the long-run, we do still know that long-term fluctuations occur. For example, a report titled "A 1500-year reconstruction of annual mean temperature for temperate North America on decadal-to-multidecadal time scales" was published by the Instituted of Physics in London. This report shows that the earth is warmer than it was, e.g., in the year 1800, but quite a bit cooler than it was in the year 800.
There do seem to have been eras of true global warming: several decades around the year 600 A.D., and again between the years 750 and 900. The year 1300 also seems to have been near the midpoint of several decades of global warming.
Historians have noted that a few decades of unusually warm, unusually cool, unusually wet, or unusually dry weather have been the causes for large-scale shifts in crop yields which affected the Roman Empire, or caused mass migrations by groups like the Huns, the Goths, and the Magyars. Around the year 1000 A.D., the Vikings gave up their habit of coastal raiding and settled into an agricultural lifestyle, due to a massive change in the climate.
Such significant changes in climate took place before industrialization, and so could not have been caused by human beings using fossil fuels. Steve Connor writes:
Genghis Khan owes his place in history to a sudden shift in the Asiatic climate from the cold, arid period that immediately preceded his ascent as leader of the Mongol empire, to the warmer, wetter weather that allowed his horsemen to expand out from Central Asia.
Connor, writing in the Independent newspaper, notes a multi-decade increase in rainfall:
Scientists studying ancient Siberia pine trees in central Mongolia that date back nearly 2,000 years believe that Khan’s rise to power coincided precisely with a period of unusually heavy rainfall over a couple of decades which allowed the arid grasslands of the Asian Steppe to flourish.
Both long-run and short-run warming and cooling trends are possible, in the present and in the past. It's difficult to know if we are in the midst of some long-run trend at the present; the patterns become clearer in hindsight. Short-run or localized trends shed no light on planet-wide trends.
The phrases 'climate change' and 'climatic instability' were adopted when it became clear that verifying claims of warming were difficult. The earth's climate is both changing and unstable. The nature of such change and instability is twofold: first, it has endured over centuries; second, it seems unrelated to human activity.
The world has, through all recorded history, experienced climatic instability. Ancient records tell us of flood and droughts. Decades-long patterns of unusually harsh winters follow decades of unusually mild winters. Winds change directions, the jet stream wobbles a bit northward and then a bit southward. One year has many hurricanes, the next year few.
To say that the earth's climate is changing is again imprecise. The planet's weather is in constant flux, and has always been so. It is changing inasmuch as it is its nature to be constantly changing; but it is not changing its essential nature. Indeed, the only real change possible would be if the weather stopped changing.
The questions which attract attention to the climate are these: is it possible that human activity impacts weather patterns? If there is some shift in the earth's climate, might some of it be due to industrialization? Are fossil fuels creating 'greenhouse' gasses which are causing global warming? Does carbon-based combustion and carbon dioxide emission have the power to affect the climate? Is methane increasing and thereby disrupting the climate?
The earth's climate has demonstrated, over millennia, drastic swings and statistical 'outlier' periods, which make it unpredictable even to modern meteorologists. Understanding cause and effect within the global climate is not obvious, and is often counter-intuitive.
For example, during the early years of the Cold War, various nations conducted hundreds of nuclear bomb tests. These tests released radiation and also hurled large amounts of fine-particle dust into the atmosphere. Linked to a number of detrimental health effects, these bomb tests have not, however, been linked to any measurable climatic effects.
By contrast, a single event, the volcanic eruption of Krakatoa, caused measurable weather changes around the world for several years. Why Krakatoa caused such a change, and yet the nuclear bomb tests did not, remains unclear. Our understanding of the climate is still partial, and does not allow us to confidently assert that human use of fossil fuel causes climate change.
The amount of data needed - precise measurements over many parts of the earth's surface stretching back centuries and millennia - is both staggering and lacking. Any confident statement that global warming is underway, or that human activity is changing the world's climate, is at least premature, and possibly unverifiable.
