The world’s leaders are touting a victory over the 2015 agreement in Paris to reduce carbon dioxide emissions and so to prevent dangerous climate change.
A number of scientists have spoken out, saying that the Paris agreement is merely political theater and will do little to reduce global warming. Ironically, many scientists on both sides of the climate debate agree regarding the potential efficacy of the Paris agreement to alter the trajectory of climate change: i) scientists who view the proposed emissions reductions as insufficient to significantly alter the warming trajectory, and ii) scientists who regard climate variations to be relatively insensitive to carbon dioxide emissions and hence insensitive to such policies.
The 2013 Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) made the dire projection that we can expect about 4 degrees Celsius of warming by the end of the 21st century if carbon dioxide emissions are not reduced. To assess the credibility of this prediction in terms of the actual trajectory of the 21st century climate, it is important to point out that the global climate models cannot predict future major volcanic eruptions or solar cycles, and do not adequately predict the long-term oscillations in the ocean.
What is the global warming hiatus, and why does it matter?
The credibility of the IPCC’s projections of 21st century climate has been called into question by a slowdown of the rate of warming in the early 21st century, relative to a more rapid rate of warming in the last quarter of the 20th century. This slowdown is referred to as the “global warming hiatus.”
The 2013 IPCC assessment made the following statement: “the rate of warming over the past 15 years . . . is smaller than the rate calculated since 1951.” Most significantly, the observed rate of warming in the early 21st century was slower than climate model predictions. The growing discrepancy between climate model predictions and the observations has raised serious questions about the climate models that are being used as the basis for national and international energy and climate policies.
A comparison of three global surface temperature datasets is shown in Graph 1 for the period since the 1990s. The data set with the largest trend since 1998 (0.1 C per decade) is the new NOAA data set (the black line), which has a trend that is 50 per cent greater than some of the other data sets. However, even the larger NOAA trend is just below the lower end of the climate model projections for the early 21st century warming of 0.11 to 0.43 C per decade.
The warming hiatus is most clearly revealed in the global satellite data sets of lower atmospheric temperature in Graph 2.
Scientists disagree on the reasons for the discrepancies between the variations of surface temperature and the lower atmospheric temperatures. The presence of El Nino and La Nina events compounds the difficulty in interpreting trends. Scientists working with the global surface temperature datasets have predicted an 85 per cent probability that 2015 will be the warmest year on record. However, scientists working with the satellite data of lower atmospheric temperatures do not foresee 2015 as being among the warmest years.
Scientists continue to debate these temperatures and investigate the reasons for discrepancies among the data sets. It will likely be five years into the future before we have the perspective to identify whether the warming hiatus has ended, or whether the warming in 2015 from the large El Nino event will be followed by several cool years, as is often the case following El Nino events.
What are the implications of the warming hiatus for our understanding of how much of the recent warming has been caused by humans? The significance of a reduced rate of warming since 1998 is that during this period, 25 per cent of human emissions of carbon dioxide have occurred.
The key conclusion of the 2013 Assessment Report of the IPCC is that it is extremely likely that more than half of the warming since 1950 has been caused by humans, and climate model simulations indicate that all of this warming has been caused by humans.
Global surface temperature anomalies since 1850 (from the Hadley Centre and the UK Climate Research Unit) are shown in Graph 3.
If the warming since 1950 was caused by humans, what caused the warming during the period 1910-1945? In fact, the period 1910-1945 comprises over 40 per cent of the warming since 1900, but is associated with only 10 per cent of the carbon dioxide increase since 1900. Clearly, human emissions of greenhouse gases played little role in causing this early warming. The mid-century period of slight cooling from 1945 to 1975 ‚Äì referred to as the “grand hiatus” ‚Äì also has not been satisfactorily explained.
Apart from these unexplained variations in 20th century temperatures, there is evidence that the global climate has been warming overall for the past 200 years, or even longer. While historical data becomes increasingly sparse in the 19th century, the Berkeley Earth Surface Temperature Project has assembled the available temperature data over land, back to 1760 in Graph 4.
