A guest post from Prof Piers Forster, a professor of physical climate change at the University of Leeds and recipient of the Royal Society Wolfson Merit Award. Forster is also a Carbon Brief contributing editor.

The world has spoken. It wants to limit future temperatures rises to 1.5C above historic levels. To achieve this everything must change.  

The twelfth of December 2015 may well be remembered as the day the human race came together and saved the world. Old differences between rich and poor, west and east were laid aside. Unbeknownst to anyone, six months ago and in secret, the sinking Marshall islanders started to raise an army of more than 100 ambitious nations that rose above the flotsam and jetsam of self-interest and created a stronger climate agreement than anyone thought possible.

The Paris agreement aims to hold the increase in the global average temperatures to “well below 2C above pre-industrial levels” and to “pursue efforts to limit the temperature increase to 1.5C”. It also requires parties to produce audited emission-reduction commitments ratcheting up every five years, and delivers a “floor” of $100 billion per year of financing up to 2025.

So unexpected was this that we climate scientists were caught napping. Before Paris, we all thought 2C was a near-impossible target and spent our energies researching future worlds where temperatures soared. In fact, there is still much to discover about the specific advantages of limiting warming to 1.5C, and the plausible social and economic pathways that might keep us under this limit.

However, the research there is points towards some important benefits of a 1.5C warming limit when compared to 2C. As described in a study published last week by Nature Geoscience, the pace of climate impacts can increase drastically at or before 2C of warming. For example, as you can see in the graphs below, the proportion of coral reefs and land for staple crops affected by warming increases rapidly between 1.5C and 2C.

Climate change impacts on various sectors per degree of warming. Shown as impact by proportion of each sector affected (left) and as a proportion of the total maximum potential impact that climate change could have (right). In other words: the left-hand chart shows how much of a sector is affected by climate change, and the right-hand chart shows how severe that impact is. Source: Ricke et al. (2015).

Some have derided the 1.5C target as a pipe dream, given that current national pledges to reduce carbon dioxide emissions – known as Intended Nationally Determined Contributions (INDCs) – could bring us closer to 3C. However, the limited research that does exist suggests that it is possible to overshoot 1.5C and return below it by 2100. And the figure below illustrates how a five-year ratchet mechanism of increasingly ambitious INDCs could deliver a temperature close to 1.5C by 2100.

Illustrates how a five-year ratchet mechanism could reach the long-term temperature target ambitions by approaching net-zero emissions of greenhouse gases after 2050. Chart shows CO2 emissions (blue bars), anthropogenic sinks of greenhouse gas emissions (red bars), example INDC trajectories (green lines), and the temperature change (black line). Bars above zero (on left-hand axis) indicate emissions into the atmosphere, while bars below zero indicate gases being removed from the atmosphere (negative emissions). Emissions and temperature change are taken from the IMAGE SSP1-2.6 model in the Shared Socioeconomic Pathway (SSP) version 1.0 database. Figure by Piers Forster.

In many ways, the answer to achieving a 1.5C pathway is simple: we need everything and we need it now. Historic emissions are still warming the oceans so even if emissions were to stop tomorrow we still might pass 1.5C.

Low-carbon transitions require huge changes to infrastructure and societal behaviour that don’t happen overnight. This is where the fun begins, because 1.5C means we will require everything in our arsenal: renewable energy, nuclear power, a dash from coal to gas, zero-carbon transport, energy efficiency, housing changes, low-carbon thermal heating and cooling systems. Even international aviation and shipping that were excluded from this report will need to be tackled within the next few years.

In particular, we will need large amounts of afforestation and carbon capture and storage. Given the lags in the system, even this world-transforming revolution is unlikely to be enough to prevent an overshoot lasting several decades.

Measures that have a more immediate effect on surface temperature will also be required. Mitigation of short-lived climate pollutants, principally methane from agriculture and black carbon (soot) from wood stoves and transport, can help rapidly cool the climate. The short-term cooling from such measures are rather redundant without strong CO2 mitigation at the same time, but with mitigation in place, they are the most effective way of shaving-off overshooting scenarios.

You can see this in the chart below. The red line shows the likely temperature response from the IPCC’s RCP2.6 scenario, where emissions are curbed to keep global average temperature rise to within 2C above pre-industrial levels by 2100. The green line is the same scenario, but with additional reduction in methane. The blue line shows a scenario similar to the current commitments of the INDCs as they stand.

Compares two scenarios that get close to the 2100 target with a scenario illustrative of current commitments. The Current Commitment is illustrated by the IMAGE-SSP1 Ref scenario. The 2C overshoot is illustrated by the REWIND-MAGPIE-SSP5 2.6 scenario. The 2C limit is illustrated by the IMAGE-SSP1 2.6 scenario. This has additional methane emission reduction compared to the overshooting scenario. Scenarios are taken from the SSP database version 1. Temperatures are computed with the MAGICC6 model. Figure by Piers Forster.

The emission target articulated in Article 4.1 of the Paris Agreement illustrates the level of change needed if the world is serious about halting global warming at such a level, ”to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century”. This wording is clear that atmospheric greenhouse gas concentrations should stop growing soon after 2050, but the meaning of “sinks” is usefully ambiguous.

Sinks can be interpreted as either anthropogenic (large scale carbon capture and storage) and/or natural (relying of forests). It also allows potential trade between greenhouse gases and avoids “zero net carbon emissions” – language that was obviously a line in the sand for some countries as it could imply near-zero fossil fuel use.

After Paris there is lots of work to do to turn the agreement into actions. As scientists, we will also need to work hard to help policy makers choose effective transition pathways and better determine the impacts of unavoidable further global warming.

I for one can’t wait to roll up my sleeves and get started on researching and planning for this brave new world.

Main image: Civil Society helps COP21 Choose the right road to 1.5C degrees. Credit: Takver/Flickr.