In 1896 a Nobel laureate, one Svante August Arrhenius of Sweden, produced a paper on the effects of Carbon Dioxide emissions caused by human activity. He observed that the amount of CO2 in the atmosphere had risen and was continuing to rise. Whatever else we consider, we must realize that the changes have been occurring for over a century. To say that the past fifty years do not provide sufficient information is to ignore over half of the available data. In the 1930s, my family realized that the United States and North Atlantic region had warmed significantly during the previous half-century. In stories of World War one and two, my parents, grandparents, and at least one great uncle informed me that the world was getting warmer. Pictures in the local Museum showed the river covered with ice, with horses and wagons crossing on the frozen surface. I've never seen that river freeze, not in fifty years of memory. Dr Timothy Ball (Canada) is convinced that Global Warming is a myth, stating that "Global Warming, as we think we know it, doesn't exist." I wonder if he is aware that the Northern Glaciers in his native land are shrinking at an alarming rate. I've been to Alaska, where there is much less ice than in the photographs from earlier times. Scientific opinion notwithstanding, we live in a warmer world than our grandparents did.
From Thomas J Crowley we have "The combination of a unique level of temperature increase in the late Th century and improved constraints on the role of natural variability provides further evidence that the greenhouse effect has already established itself above the level of natural variability in the climate system." Another paper observes that comparison between atmospheric Carbon 14 and changes in climate has shown a first-order relationship between a variable Sun and climate. These two observations leave open the possibility that a solar change is attempting to cool the earth, while our 'Greenhouse effect' is raising it. This would help explain why the effect is less than predicted by Dr. Arrhenius. It is worthwhile to note that the rate of CO2 production has rapidly increased. Dr. Arrhenius expected CO2 doubling to take about 3000 years instead of the 100 years or less now predicted. From the US governmental Agency the EPA we learn, "Scientists have been able to piece together a picture of the Earth's climate dating back decades to millions of years ago by analyzing a number of surrogate, or "proxy," measures of climate such as ice cores, boreholes, tree rings, glacier lengths, pollen remains, and ocean sediments, and by studying changes in the Earth's orbit around the sun." They cite Orbital variations, Solar fluctuation and Greenhouse gas emissions as among the driving factors in Climate change. They note that Volcanoes produce CO2, and may have contributed to past 'warm periods' in geological history. Incidentally, human activities now emit 130 times as much CO2 as volcanoes.
In the 1970s, while I was a sailor in southeast Asia, analysis of Northern Hemisphere weather statistics showed that a cooling trend had begun in the 1940s. I was in the jungle and didn't notice much cooling, but then, no one really cared at that time. We were still pondering the damage our aerosol cans had done, and how much of the Ozone layer would actually be lost. Studies of ancient climates show that astronomical cycles at least partially set the timing of the ice ages. Apparently climate is so delicately balanced that almost any small perturbation may set off a great shift. A wide variety of evidence supports the global existence of a "Little Ice Age" between about 1500 and 1850. Average temperatures were possibly up to 2F colder than today, but varied by region. As previously noted, from 1850 until the present we have been warming the earth via human activity. According to the climate indicators which have been, and are still being, studied, we are at about the warmest period on earth for the last thousand or so years. Scientists will only speculate on the last 400 or so years, due to the lack of verifiable evidence before that time, but what evidence exists shows a cooler earth during the Norman Conquest. Another consideration is Abrupt climate changes which occur when a threshold in the climate system is crossed. A trigger causes the climate to rapidly shift from one state to a new, different one. Crossing thresholds in the climate system may lead to large and widespread consequences in a very short time. Again from the EPA report, "The potential for abrupt climate changes cannot be predicted with confidence; however, abrupt climate changes are an important consideration because, if triggered, they could occur so quickly and unexpectedly that human or natural systems would have difficulty adapting to them (NRC, 2002)."
Finishing the argument is a simple task, because it is just a suggestion that we not rely entirely on the scientific method in this case. Waiting to see what will happen and then formulating a theory as to why could lead to many New York City residents having to learn to swim. Holland would need to upgrade the dikes which are so famous in that country. Florida would need to be largely evacuated. I've noticed that the majority of the earth's population tends to live near the ocean, for whatever reason. If the ocean comes calling, we might look back on the recent Tsunami for estimates of the possible damage. On a lighter note, if the melting Glaciers dump enough fresh, cold water into the oceans, there might be a major shift in the currents. That has been known to cause Glacier formation and lots of snow in the South.