Strong chemistry‐climate feedbacks in the Pliocene

Unger, Nadine; Yue, Xu

doi:10.1002/2013gl058773

Cited by 43 publications

(48 citation statements)

References 62 publications

(73 reference statements)

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“…Our work thus suggests that SOA reductions may have amplified regional warming in the present but minimized regional cooling at the LGM relative to the preindustrial. Results from these sensitivity studies, however, underscore the large uncertainties in current model estimates of SOA radiative forcing across long timescales (e.g., Scott et al, 2014;Unger and Yue, 2014;Unger, 2014a). Unlike SOA, we find that changes in tropospheric mean ozone burdens relative to the preindustrial are insensitive to the uncertainties in isoprene emissions and photochemistry tested in this study.…”

Section: P Achakulwisut Et Al: Uncertainties In Isoprene Photochemisupporting

confidence: 41%

Uncertainties in isoprene photochemistry and emissions: implications for the oxidative capacity of past and present atmospheres and for climate forcing agents

et al. 2015

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Abstract. Isoprene and its oxidation products are major players in the oxidative chemistry of the troposphere. Current understanding of the factors controlling biogenic isoprene emissions and of the fate of isoprene oxidation products in the atmosphere has been evolving rapidly. We use a climate-biosphere-chemistry modeling framework to evaluate the sensitivity of estimates of the tropospheric oxidative capacity to uncertainties in isoprene emissions and photochemistry. Our work focuses on two climate transitions: from the Last Glacial Maximum (LGM, 19 000-23 000 years BP) to the preindustrial (1770s) and from the preindustrial to the present day (1990s). We find that different oxidants have different sensitivities to the uncertainties tested in this study. Ozone is relatively insensitive, whereas OH is the most sensitive: changes in the global mean OH levels for the LGM-to-preindustrial transition range between −29 and +7 % and those for the preindustrial-to-present-day transition range between −8 and +17 % across our simulations. We find little variability in the implied relative LGMpreindustrial difference in methane emissions with respect to the uncertainties tested in this study. Conversely, estimates of the preindustrial-to-present-day and LGM-to-preindustrial changes in the global burden of secondary organic aerosol (SOA) are highly sensitive. We show that the linear relationship between tropospheric mean OH and tropospheric mean ozone photolysis rates, water vapor, and total emissions of NO x and reactive carbon -first reported in Murray et al. (2014) -does not hold across all periods with the new isoprene photochemistry mechanism. This study demonstrates how inadequacies in our current understanding of isoprene emissions and photochemistry impede our ability to constrain the oxidative capacities of the present and past atmospheres, its controlling factors, and the radiative forcing of some short-lived species such as SOA over time.

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Section: P Achakulwisut Et Al: Uncertainties In Isoprene Photochemisupporting

confidence: 41%

Uncertainties in isoprene photochemistry and emissions: implications for the oxidative capacity of past and present atmospheres and for climate forcing agents

et al. 2015

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“…in the size dependency, and by variations in the degree of passive vs. active uptake of CO 2 by the alkenone-producing coccolithophorids (Bolton and Stoll, 2013). Furthermore, van de Wal et al (2011) already showed that the relationship of CO 2 to temperature change during the last 20 Myr is opposite in sign for alkenone-based CO 2 than for other approaches.…”

Section: Detecting Any State Dependency In S [Co 2 Li]mentioning

confidence: 97%

“…CH 4 , N 2 O, or aerosols. For the Pliocene, strong chemistry-climate feedbacks have been proposed (Unger and Yue, 2014), suggesting high ozone and aerosol levels and potentially high CH 4 values. This implies that the relationship of CO 2 to other forcing agents might have been different for the cold climates of the late Pleistocene than for the warm climates of the Pliocene.…”

Section: Martínezmentioning

confidence: 99%

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On the state dependency of the equilibrium climate sensitivity during the last 5 million years

et al. 2015

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Abstract.It is still an open question how equilibrium warming in response to increasing radiative forcing -the specific equilibrium climate sensitivity S -depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO 2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO 2 radiative forcing depends on the CO 2 data set used. This nonlinearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of icesheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO 2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S [CO 2 ,LI] , for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S [CO 2 ,LI] is on average ∼ 45 % larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6-5 Myr BP) the CO 2 data uncertainties prevent a well-supported calculation for S [CO 2 ,LI] , but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 Myr BP), the specific equilibrium climate sensitivity, S [CO 2 ,LI] , was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO 2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.

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“…1), nor by a higher Earth System Sensitivity 10 to CO 2 forcing as suggested in an accompanying News & Views 13 . At its heart lies identifying mechanisms that can support weak temperature gradients 1,11 , which may be rooted in models' relatively weak meridional SST gradient reduction due to unresolved climate feedbacks [14][15][16] . The Pliocene puzzle becomes even more concrete when looking further back into the Early Pliocene (4-5Ma) as SST proxies from around the globe indicate a further weakening of SST gradients 1 , whereas constraining warm pool SST changes remains difficult since they fall within the uncertainty of paleo proxies.…”

mentioning

confidence: 99%