The Climate Summit in Paris ended Dec 12 amid global fanfare and nearly as widespread skepticism, for the historic agreement.

The fanfare celebrates the Paris Agreement (see here for the document itself and here for a synopsis), the first (ever!) universal agreement to address climate change. This agreement, many suggest, signals a turning point in global recognition of the risks and realities of climate change . “Nobody questions what the science says,” a quote from Miguel Arias Cañete, the European Union’s climate action and energy commissioner, that represents a big (and important) change from previous summits. Some even herald the agreement as marking the “end of the fossil fuel era.”

The skepticism focuses on whether the non-binding commitments each country has now agreed upon will be realized and whether they are even enough to avert the worst of the climate related changes predicted by scientists.

Rather than weigh in on the agreement itself (which a lot of great minds are already doing, see here for just one example), I want to look at the goal of the agreement – keeping the average global temperature limited to 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-Industrialized temperatures.

So why 2 degrees Celsius? Why is the goal just a matter of degrees?

Limiting global average temperature is a goal for the international community, because temperature is a key factor in climate change (for a refresher on average global temperature and climate, please see my previous post here. Global temperatures have always varied year to year, but the long-term trend was very stable. Over the past 10,000 years (or most of human civilization), the temperature varied within 1 degree Celsius, with abnormalities explained by natural events such as volcanic eruptions (see this great PBS News Hour video here).

Global average temperature varies in tandem with variation of carbon dioxide (CO2) and other greenhouse gases in the atmosphere due to the Greenhouse Effect.

Essentially, when sunlight hits the earth, some energy is reflected away but some is absorbed by the earth. As this absorbed energy is released from the earth as heat, the greenhouse gases in the atmosphere retain some of the energy and prevent its release into space, warming the planet, much like how glass works in a literal greenhouse.

The Greenhouse Effect is a big part of what makes planet Earth habitable for its diversity of life-forms!

The problem is that starting in the industrial era, humans have been releasing more CO2 and other greenhouse gases into the atmosphere, increasing the strength of the greenhouse effect…

…. and resulting in a rise in global average temperature.

These changes have strong impacts on natural processes, including the timing of natural events (known as phenology) like spring flowers and autumnal leaf color changes (see my post here) as well as changes in the level of rain (related to both floods and droughts), extreme weather events (like hurricanes and typhoons), and sea level changes due to melting of glaciers and ice caps.

It is these extreme events that the Paris Agreement is trying to avoid with its goal of preventing a 2 degree Celsius global temperature rise.

The 2 degree limit itself may have more political import than scientific merit, but the general idea is to avoid a tipping point, or a level of warming beyond which there is no return.

Scientists use robust models to make these estimations and they predict that there is a threshold to earth’s resilience, probably between 1-3 degrees warmer than pre-industrial averages for moderate climate impacts. In this case even moderate changes are likely to result in sea level rise of several feet (impacting many of the world’s smallest island nations and biggest cities), extreme droughts and heat-waves (threatening food supplies across the globe), and a myriad of other risks including extreme weather events such as typhoons and hurricanes.

And beyond this threshold, increasing temperatures will trigger positive feedback loops that will accelerate the warming process and make the above risks go from possible or likely to inevitable.

In 2015 the urgency of global agreements and actions on climate change has never been greater, as 2015 is on track to be the warmest year on record, bringing us past the 1 degree Celsius mark already.

Hence the both skepticism and excitement for the Paris Agreement! Never has it been more critical to make significant changes, but the obstacles are significant.

Author note: It is hard to not be overwhelmed by the facts presented above (but here is a more sunny take on the issue), it happens to me too and I’ve been working on the issue of climate change both as a scientist and sometime activist for over 10 years. But my goal here is not to depress you, but to galvanize you! New Years is coming up and now is a good time to think about how we all can contribute to helping exceed the goals of the Paris Agreement, so that the global temperature does not exceed 2 degrees.

Please share your recommendations and resolutions in the comments below!