Ever since global leaders came to an agreement in Copenhagen that “an increase in global temperature should be below 2 degrees Celsius ” in 2009, there have been heated discussions over whether the so-called 2-degree goal is appropriate, realistic or sufficient as a means of avoiding the worst climate scenario. CNN even set up a “2-degrees series” on its opinion section to thread the goal through energy and climate issues .
Criticism is generally two-pronged; first is about ‘what’—the 2 degrees as an indicator. In 2014, Victor and Kennel asserted in Nature that the world should “ditch the 2-degree warming goal,” which is “effectively unachievable and impractical .” There are better indicators that reflect the state of global warming, they argue, such as ocean heat content. Second is about ‘how’—ways to achieve the 2-degree goal and their effectiveness. As there is no specific guidance for governments and decision-makers about exactly how to achieve this goal, opponents say taking immediate measures, such as carbon tax, is a better-defined goal to reduce global carbon emissions. (Selecting an appropriate level of discount rate is another matter, of course.)
Nonetheless, the temperature goal was repeatedly confirmed in the following COPs, and the countries around the world came up with an Intended Nationally Determined Commitment (INDC) based on this goal. The primary reason for the persistence is that the goal was not set arbitrarily; it was set in temperature terms for a reason. Temperatures are quantitative as well as “linked to radiative forcing,” and reflective of risks and impacts of the globe . While experts acknowledge that no single metric is sufficient to represent the level of threats of climate change, it serves as an ‘anchoring device ’ on which mitigation efforts converge.
While the 2-degree goal does not directly lay out cap and trade or carbon taxes that would bring significant GHG reductions, it allows decision-makers to achieve the goal with the flexibility to choose the means that best fit them. While carbon taxes do not guarantee the desirable outcome on its own, they can be a powerful policy tool if pursued under the 2-degree goal.
One last thing: there is a hidden third fork of criticism; about “2” degrees. Recall that the 2-degree goal is not to stabilize the temperature “at” 2 degrees, but “within” 2 degrees. Some island nations like Antigua coined a catchphrase “One-point-five to stay alive.” Half degree seems nothing, but experts say that the projected risks between 1.5 and 2 degrees vary significantly in highly temperature-sensitive systems, such as polar regions and tropics, and in terms of food security . Maybe the 2-degree goal is not something we chose, but something that we settled for. The global average temperature change is likely to exceed 1.5 °C by 2100 relative to pre-industrial levels anyway .
 Victor, D. G. and Kennel, C. F., October 2014, Ditch the 2°C Warming Goal. Nature. Vol. 514.
 RealClimate, October 2014. Limiting Global Warming to 2°C—Why Victor and Kennel Are Wrong. Retrieved from http://www.realclimate.org/index.php/archives/2014/10/limiting-global-warming-to-2-c-why-victor-and-kennel-are-wrong/
 Geden, O. and Beck, S., September 2014. Renegotiation the Global Climate Stabilization Target. Nature Climate Change. Vol. 4.