Climate Change Caused by Humans
Climate Change: The Smoking Guns for Humans

It is well established that increasing greenhouse gases such as carbon dioxide and methane will warm the earth's lower atmosphere (troposphere). It is also well established that present day concentrations of these two greenhouse gases have been measured at higher levels than those observed over the previous hundreds of thousands of years. Furthermore, the concentration rate is increasing. Where is all of this carbon dioxide coming from? According to Mann & Kump (2009), the IPCC (2007), Ghosh & Brand (2003), Ruddiman (2001), and others:

Volcanoes also outgas carbon with low 13C/12C ratios. Low 13C/12C ratios from volcanic activity have been observed by Cartigney et al. (2009), Mastalerz, de Lange, Dählmann, & Feseker (2007), Castrillo et al. (2004), Erdelyi, Richter, & Title (2002), and others. So could volcanoes be causing the observed decreasing atmospheric 13C/12C ratios? NO.

As discussed in Global Warming Misinformation - Volcanoes Emit More CO2 Than Humans, emissions from humans dwarf those of volcanoes. According to the IPCC (2007) WGI Report Chapter 2:

Furthermore, according to the United States Geological Survey (2009):

Carbon-13 Carbon-12 ratios decreasing
Figure 3.1: Since the Industrial Revolution, 13C/12C has been decreasing (Zhao et al., 2006)

Carbon-13 Carbon-12 ratios decreasing
Figure 3.2: More recent CO2 global emissions vs. 13C/12C ratios (IPCC, 2007)

Figures 3.1 and 3.2 show that as global CO2 emissions are rising, the 13C/12C ratio is decreasing (larger negative values) due to human emissions of carbon dioxide.

14C is radioactive and is measured in tree ring data. Measured decreases of 14C/12C since the Industrial Revolution show that 12C is increasing (Figure 3.3, Mann 2009). This increase cannot be from volcanoes or deep oceans – only from fossil fuel emissions and land clearance. Because of above ground atomic bomb testing in the late 1940s continuing through 1963, 14C concentrations greatly increased. Therefore, 14C/12C ratios since the late 1940s cannot be used to infer carbon increases from human emissions.

Carbon-14 Carbon-12 ratios decreasing
Figure 3.3: Since the Industrial Revolution, 14C/12C has been decreasing

Figure 3.4 (Ruddiman, 2001) shows a time series of the two main sources of anthropogenic carbon in the atmosphere. One can see that since the Industrial Revolution carbon emissions have dramatically increased.

Measured Carbon Emissions vs. Time
Figure 3.4: Measured Carbon Emissions vs. Time

Figure 3.5 shows that since 1751 approximately 329 billion tons of carbon have been released to the atmosphere from the consumption of fossil fuels and cement production. Half of these emissions have occurred since the mid 1970s. The 2006 global fossil-fuel carbon emission estimate, 8230 million metric tons of carbon, represents an all-time high and a 3.2% increase from 2005. (Boden, T.A., G. Marland, and R.J. Andres, 2009)

Carbon-14 Carbon-12 ratios decreasing
Figure 3.5: Since the Industrial Revolution, 14C/12C has been decreasing

Figure 3.6 (Ruddiman, 2008) shows where the carbon emissions end up. Fossil fuel burning and land clearance are sources of carbon while the oceans and land (primarily due to vegetation) are sinks for carbon. One should note that the measured amount of carbon in the atmosphere is only 55% of what is actually being emitted into the air.

Where do Carbon Emissions Go?
Figure 3.6: Where do Carbon Emissions Go?

Figure 3.7 (IPCC, 2007) reveals that the rate of carbon sources is larger than the rate of carbon sinks which is why carbon in the atmosphere has been dramatically increasing. Plants and the oceans are not able to keep pace with industrial output and land clearance.

Carbon Source and Sink Rate
Figure 3.7: Carbon Source and Sink Rate

Carbon dioxide levels in the atmosphere are currently at unprecedented levels and the source for this additional carbon is primarily from human activities. There are no other known plausible arguments to explain modern carbon levels without humans being the main cause of the increase.

Could the increasing carbon in the oceans and ocean acidification be due to volcanic outgassing? NO. Figure 3.8 (Ruddiman, 2001) shows that deep ocean 13C/12C ratios (δ13C) have varied only slightly (between -1.2 and +1) over the last two million years. Submarine volcanoes emit carbon with δ13C values of -4 or less so their contribution must be insignificant. IPCC WGI Chapter 7 (2007) and Ruddiman (2001) show that carbon stores in the ocean are primarily driven by alternating phases of glaciations. During glacial climates, there is less land vegetation so the 12C would have to be stored in the deep ocean resulting in higher δ13C values. During warmer climates there is more land vegetation, so more 12C would leave the oceans to be stored inside the vegetation. This process lowers δ13C values in the oceans.

13C in Deep Oceans Over Time
Figure 3.8: Deep water δ18O and δ13C values in the past 2 million years

Figure 3.9 (IPCC, 2007) is arguably the "smoking gun" for the anthropogenic (human) cause of climate change. As the figure clearly illustrates, when climate models consider only natural causes of climate change (blue color), the actual recorded climate (black line) cannot be predicted. However, when human forcing is added to natural forcing the pink curve matches very well with the observed climate across the globe. Simply stated, the climate observed today could NOT have happened if humans did not exist - it is impossible for natural forces alone to have caused today's climate. Furthermore, in the past few decades, climate models have shown that human factors are the dominant cause of climate change.

Climate models with and without human acitivities
Figure 3.9: Climate models with and without human activities

Tropospheric warming with stratospheric cooling is essentially another "smoking gun" for anthropogenic global warming. There are no known natural forcing mechanisms that result in this coupling. Solar forcing, cloud cover, ENSO, PDO, NAO, etc. cannot explain a cooler stratosphere even when ozone depletion is accounted for. Increasing greenhouse gases explain this coupling very well and climate models predict a warmer troposphere and a cooler stratosphere with increased greenhouse gases.

10 Indicators of a Human Fingerprint on Climate Change:

Human Fingerprints
Figure 3.10: 10 Indicators of a Human Fingerprint on Climate Change (Cook, 2010)

"The NOAA State of the Climate 2009 report is an excellent summary of the many lines of evidence that global warming is happening. Acknowledging the fact that the planet is warming leads to the all important question - what's causing global warming? To answer this, here is a summary of the empirical evidence that answer this question. Many different observations find a distinct human fingerprint on climate change" - John Cook at Skeptical Science

If increases in CO2 are not causing modern day global warming then two things must be true:

  1. Something unknown is suppressing the well-understood greenhouse effect (and doing so during massive increases in GHGs).
  2. Something unknown is causing the warming that mirrors the GHE.
So we can accept what we know to be true (AGW) or we accept two unknowns.

Next: Natural Causes of Climate Change



Scott A. Mandia
Professor - Physical Sciences
T-202 Smithtown Sciences Bldg.
S.C.C.C.
533 College Rd.
Selden, NY 11784
(631) 451-4104
mandias@sunysuffolk.edu
http://www2.sunysuffolk.edu/mandias/

Last updated: 08/10/10