Matthew P. Humphreys

Climate sensitivity and the rate of ocean acidification: future impacts, and implications for experimental design

&NA; The global mean surface temperature and partial pressure of carbon dioxide (CO2) are increasing both in the atmosphere and ocean. Oceanic CO2 uptake causes a decline in pH called ocean acidification (OA), which also alters other biologically important carbonate system variables such as carbonate mineral saturation states. Here, we discuss how a “temperature buffering” effect chemically links the rates of warming and OA at a more fundamental level than is often appreciated, meaning that seawater warming could mitigate some of the adverse biological impacts of OA. In a global mean sense, the rate of warming relative to the CO2 increase can be quantified by the climate sensitivity (CS), the exact value of which is uncertain. It may initially appear that a greater CS would therefore reduce the negative influence of OA. However, the dependence of the rate of CO2 increase on the CS could enhance, nullify or even reverse the temperature buffering effect, depending upon the future trajectory of anthropogenic CO2 emissions. Regional deviations from the global mean seawater temperature and CO2 uptake trends could modulate local responses to OA. For example, mitigation of OA impacts through temperature buffering could be particularly effective in the Arctic Ocean, where the surface seawater warming rate is greater than the global mean, and the aqueous CO2 concentration might increase more slowly than elsewhere. Some carbonate system variables are more strongly affected than others, highlighting the need to develop a mechanistic understanding of precisely which variables are important to each biogeochemical process. Temperature buffering of the marine carbonate system should be taken into account when designing experiments to determine marine species and ecosystem responses to warming and OA, in order that their results accurately reflect future conditions, and therefore can generate realistic predictions when applied to Earth system models.

Multidecadal accumulation of anthropogenic and remineralized dissolved inorganic carbon along the Extended Ellett Line in the northeast Atlantic Ocean

Marine carbonate chemistry measurements have been carried out annually since 2009 during UK research cruises along the Extended Ellett Line (EEL), a hydrographic transect in the northeast Atlantic Ocean. The EEL intersects several water masses that are key to the global thermohaline circulation, and therefore the cruises sample a region in which it is critical to monitor secular physical and biogeochemical changes. We have combined results from these EEL cruises with existing quality-controlled observational data syntheses to produce a hydrographic time series for the EEL from 1981 to 2013. This reveals multidecadal increases in dissolved inorganic carbon (DIC) throughout the water column, with a near-surface maximum rate of 1.80 ± 0.45 μmol kg -1 yr -1 . Anthropogenic CO 2 accumulation was assessed, using simultaneous changes in apparent oxygen utilization (AOU) and total alkalinity (TA) as proxies for the biogeochemical processes that influence DIC. The stable carbon isotope composition of DIC (δ 13 C DIC ) was also determined and used as an independent test of our method. We calculated a volume-integrated anthropogenic CO 2 accumulation rate of 2.8 ± 0.4 mg C m -3 yr -1 along the EEL, which is about double the global mean. The anthropogenic CO 2 component accounts for only 31 ± 6% of the total DIC increase. The remainder is derived from increased organic matter remineralization, which we attribute to the lateral redistribution of water masses that accompanies subpolar gyre contraction. Output from a general circulation ecosystem model demonstrates that spatiotemporal heterogeneity in the observations has not significantly biased our multidecadal rate of change calculations and indicates that the EEL observations have been tracking distal changes in the surrounding North Atlantic and Nordic Seas.

Controls on Open‐Ocean North Atlantic ΔpCO2 at Seasonal and Interannual Time Scales Are Different

The North Atlantic is a substantial sink for anthropogenic CO2. Understanding the mechanisms driving the sinks variability is key to assessing its current state and predicting its potential response to global climate change. Here we apply a time series decomposition technique to satellite and in situ data to examine separately the factors (both biological and nonbiological) that affect the sea‐air CO2 difference (ΔpCO2) on seasonal and interannual time scales. We demonstrate that on seasonal time scales, the subpolar North Atlantic ΔpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics. However, the same factors do not necessarily control ΔpCO2 on interannual time scales. Our results imply that the mechanisms driving seasonal variability in ΔpCO2 cannot necessarily be extrapolated to predict how ΔpCO2, and thus the North Atlantic CO2 sink, may respond to increases in anthropogenic CO2 over longer time scales.

Characterization of a Time-Domain Dual Lifetime Referencing pCO2 Optode and Deployment as a High-Resolution Underway Sensor across the High Latitude North Atlantic Ocean

The ocean is a major sink for anthropogenic carbon dioxide (CO2), with the CO2 uptake causing changes to ocean chemistry. To monitor these changes and provide a chemical background for biological and biogeochemical studies, high quality partial pressure of CO2 (pCO2) sensors are required, with suitable accuracy and precision for ocean measurements. Optodes have the potential to measure in situ pCO2 without the need for wet chemicals or bulky gas equilibration chambers that are typically used in pCO2 systems. However, optodes are still in an early developmental stage compared to more established equilibrator-based pCO2 systems. In this study, we performed a laboratory-based characterization of a time-domain dual lifetime referencing pCO2 optode system. The pCO2 optode spot was illuminated with low intensity light (0.2 mA, 0.72 mW) to minimize spot photobleaching. The spot was calibrated using an experimental gas calibration rig prior to deployment, with a determined response time (τ63) of 50 s at 25°C. The pCO2 optode was deployed as an autonomous shipboard underway system across the high latitude North Atlantic Ocean with a resolution of ca.10 measurements per hour. The optode data was validated with a secondary shipboard equilibrator-based infrared pCO2 instrument, and pCO2 calculated from discrete samples of dissolved inorganic carbon and total alkalinity. Further verification of the pCO2 optode data was achieved using complimentary variables such as nutrients and dissolved oxygen. The shipboard precision of the pCO2 sensor was 9.5 μatm determined both from repeat measurements of certified reference materials and from the standard deviation of seawater measurements while on station. Finally, the optode deployment data was used to evaluate the physical and biogeochemical controls on pCO2.