Setting the Scene an Overview of non-co 2 Aviation Effects on Climate

Transcription

1 Setting the Scene an Overview of non-co 2 Aviation Effects on Climate Keith P Shine, Department of Meteorology, University of Reading k.p.shine@reading.ac.uk

2 Take-home messages Aviation non-co 2 emissions (NCE) are major contributors to aviation s total climate impact NCE have quite different characteristics to CO 2. They are generally short-lived (hours to months). Their climate effect depends sensitively on where/when the emissions occur. This is the underlying reason why smart flying might just be possible (In)famously uncertain significant recent advances in understanding NO x, water vapour, and contrail forcings, but aerosols have emerged as a more major issue For smart-flying we need to be able to place climate effect of different emissions on some kind of common scale. No obvious best-method

3 What is radiative forcing (of climate change)? Radiative forcing is the perturbation of the planetary radiation budget, in W m -2, in absence of (almost!) any other change

4 F T Why is radiative forcing useful? decades t t There is an approximate relationship between global-mean surface temperature change ΔT and global-mean radiative forcing, ΔF T F where λ is the climate sensitivity, a key parameter in understanding climate change

5 Contribution of non-co 2 forcings Aviation CO 2 RF (about 30 mw m -2 ) represents about 1.6% of the total CO 2 forcing from all human activities Lee et al., Atmos Environ 2010 When NCE are included, aviation contributes 1.3 to 14% of the total radiative forcing due to human activities

6 A generic issue where do aircraft fly? Existing aviation inventories are not in good agreement Wilcox et al. (2012) for water vapour and Skowron et al. (2013) for NOx, have shown significant sensitivity to choice of inventory

7 Lee et al., Atmos Environ 2010

8 Oxides of Nitrogen (NOx) Ozone increases positive forcing (short-lived and in the hemisphere of emission) Methane decreases negative forcing (longer-lived, global) and impacts on ozone, aerosols, H 2 O,... Multi-model comparisons have led to much improved understanding But still uncertainty over sign e.g. Pitari et al.(atmosphere, 2015) find weak negative forcing Pitari et al., (in prep, 2015) Myhre et al., (Atmos Env 2011) Net methane Short-lived ozone Net

9 Examples of altitude and latitude dependence Left global-mean forcing as a function of height of NOx emission net ozone methane methane effect on ozone Below global-mean forcing as a function of latitude of emission Also depends on weather situation... Köhler et al. (JGR, 2008) Köhler et al. (Atmos Env, 2013)

10 Lee et al., Atmos Environ 2010

11 Contrails and contrail cirrus Different types short-lived, persistent, and can evolve into clouds indistinguishable from natural cirrus Formation depends strongly on atmospheric conditions Subtle changes in aircraft altitude or location greatly influence contrail formation Contrails reflect sunlight (cooling) and trap infrared energy (warming) makes calculating the net effect more difficult and dependent on time of day/year Expanding database of measurements of contrail properties and occurrence

12 Contrails and contrail cirrus Significant recent convergence in forcing estimates (both modelled and empirical) for linear contrails and the total contrail plus contrail cirrus forcing IPCC 5 th Assessment Report report persistent (linear) contrails forcing of 10 (5 to 30) mw m -2 and total (contrail plus contrail cirrus) forcing of 50 (10 to 120) mw m -2 Burkhardt and Kärcher (Nature Climate Change 2011)

13 Contrails and contrail cirrus 250 hpa, or about10 km or FL330 Winter-time mean weather-type: strong zonal jet weather-type: strong ridge Irvine et al. (GRL 2012) Persistent contrails require (cold) air supersaturated with respect to ice Reasonably frequent at cruise altitudes (e.g. in N. Atlantic sector > 10%) Frequency is very dependent on weather situation and height, and layers are generally thin Depth of supersaturated layers over UK March-June From Rädel and Shine (QJRMetS,2010)

14 Lee et al., Atmos Environ 2010

15 Water vapour from aviation fuel Lee et al. estimated a 3 mw m -2 forcing with a 90% confidence range of 0.4 to 20 mw m -2 but forcing had not been assessed in detail Upper limit now been revised downwards significantly by Wilcox et al. (Atmos Env, 2012) to 0.9 ( ) mw m -2 - a value supported by other recent work e.g. Pitari et al. (Atmosphere mw m -2 ) Annual-mean change (ppb) in water vapour due to emissions from current aviation fleet. Wilcox et al., (Atmos Env 2012)

16 Lee et al., Atmos Environ 2010

17 Aerosols Rapidly evolving field Lee et al. assessed as rather small, but with large uncertainties Two recent estimates indicate possibly large negative forcings, because of sulphate transported from cruise level to low-altitude clouds Impact of aviation soot on (natural) cirrus clouds depends sensitively on presence of other particles. Zhou and Penner (JGR, 2014) get -350 to +90 mw m -2 but without a best estimate. Gettelman and Chen (GRL, 2013) -38±10 mw m -2 Righi et al (ACP, 2013) 2.4 to -70 mw m -2

18 Weighting non-co 2 emissions Fuglestvedt et al. (Atmos Env, 2010) How do we compare NCE with CO 2? Remains contentious - it depends on viewpoint Using a Radiative Forcing Index (Total Forcing/CO 2 forcing), the present-day effect of all past emissions, the present-day value is about 2.8 But this masks the fact that the CO 2 forcing is persistent but most of the NCE forcings are rather short-lived

19 A CO 2 multiplier? CO 2 multiplier depends on choice of metric and the time period over which the effects are looked at. This is a policy decision not a science decision! Example metrics include the Global Warming Potential (GWP) used by the Kyoto Protocol to compare the effect of different emissions And the Global Temperature-change Potential (GTP), perhaps more applicable for a target-based climate policy (e.g. keeping temperature change below 2 deg C since pre-industrial time) Use data from Fuglestvedt et al. and Lee et al. (Atmos Env. 2010) here

20 The GWP and GTP Figure from Jan Fuglestvedt, CICERO, Oslo Global Warming Potential time integrated radiative forcing following a pulse emission Global temperaturechange potential temperature change some time after a pulse emission

21 Example emission metrics for the global fleet CO 2 -equivalent emissions in 2005 GWP (20) GWP (100) GTP (20) GTP (50) CO Oxides of Nitrogen 110 to to to to -50 Water vapour Direct Sulphate Direct Black Carbon Contrails Contrail cirrus Ratio

22 Concluding remarks Significant progress in quantifying NCE effects from aviation more confidence in contrail, NOx and water vapour effects Aerosol climate impacts have emerged as a large uncertainty, causing a possibly significant cooling effect. To compare climate impact of emissions of CO 2, contrails, NOx, etc, requires some method of putting them on a common scale no obvious best method and requires user/policy choice of appropriate parameters and appropriate time horizons