To understand how Earth’s climate will change in the future, scientists need to know how much heat-trapping gas is going into the atmosphere today. However, oceans’ emissions of two major greenhouse gases, methane (CH4) and nitrous oxide (N2O), vary dramatically in time and space. With a centralized digital resource, the Marine Methane and Nitrous Oxide (MEMENTO) database, information on CH4 and N2O concentration measurements from around the globe are collected to help researchers more precisely quantify these oceanic emissions.
Oceanic CH4 can arise from shallow sediments, and both CH4 and N2O are produced by ocean-dwelling microbes. Although only a relatively small fraction of global CH4 emissions—around 2%—come from the ocean (including coastal areas), oceans are a major source for atmospheric N2O, providing around 25% of the total. When it reaches the stratosphere, N2O attacks ozone, destroying it on a global scale.
Estimates of oceanic emissions are based on extrapolations of concentrations measured at the ocean’s surface or results from model studies. For example, using the data set of Weiss et al. , Nevison et al.  calculated the first global field of surface ocean N2O concentrations to estimate the marine N2O source to the atmosphere.
However, the fluxes of N2O and CH4 can vary substantially from day to day and from place to place, meaning that even with recent improvements in measurement techniques and increased measurements, global emission estimates are still highly uncertain [see Ciais et al., 2013]. Millions of measurements taken at different times and covering the globe are needed for researchers to more precisely estimate how much gas is being emitted.
MEMENTO Ups the Game
MEMENTO, an initiative that began in 2009, is the first attempt to systematically compile all global data on oceanic CH4 and N2O measurements. It archives data taken not only at the ocean surface but also from the deep ocean. As curators of the data set, our goals are to see how oceanic concentrations of the gases vary in time and space and to provide more precise global emission estimates of oceanic CH4 and N2O to the climate research community.
MEMENTO already includes original data from more than 180 measurement campaigns, which have provided more than 20,000 CH4 and more than 100,000 N2O measurements over the past 50 years (see Figure 1 for sampling locations). These data sets include dissolved gas concentrations along with information on sampling position, sampling depth and time, and, if available, data on ocean temperature and salinity as well as oxygen and nutrient concentrations.
If available, we also include atmospheric measurements from the same campaign, such as air temperature and air pressure, usually sampled a few meters above sea level height. We also add to all submissions the contact information of the researchers who provided the data, their related publications, and if available, a link to the host center of the original data sets.
An Emphasis on Quality
We put all data submissions imported to MEMENTO through a systematic quality control procedure to guarantee that essential metadata are available and to minimize erroneous entries. If measurements lack information on sampling position, sampling time, and sampling depth (for oceanographic data), we do not import them into the database. In addition, we apply a first-order range check to all imported variables to exclude obviously incorrect data entries, such as negative concentrations, erroneous date formats, or data positioned over land.
CH4, N2O, and oxygen data are imported in their original units. In a second data-processing step, we will calculate global surface fields and depth profiles in common units. Missing temperature and salinity data will be supplied from external data sources.
A Work in Progress
We regularly update the database with newly available data sets and continuously improve it by including additional meta-information, allowing additional data formats, and implementing new data quality control criteria.
In addition, we are working closely with the recently initiated Scientific Committee on Oceanic Research (SCOR) Working Group 143, entitled “Dissolved N2O and CH4 measurements: Working towards a global network of ocean time series measurements of N2O and CH4.” As an additional quality flag for our data, we will implement standard procedures that are developed within the working group for measuring N2O and CH4.
As we expand MEMENTO, we will also build on the experiences researchers have gained from existing databases such as the Surface Ocean CO2 Atlas (SOCAT), the Global Surface Seawater Dimethylsulfide Database (GSSDD), and the Halocarbons in the Ocean and Atmosphere Database Project (HalOcAt). Specifically, we are looking to create best practices on how to structure data archives, methods for checking data quality, and ways to make data archives more user friendly.
A Resource for the Research Community
We intend for MEMENTO to serve as a living resource from which researchers can pull quality-controlled oceanic CH4 and N2O data for a variety of purposes. Researchers have already begun using the database to produce important results. For example, Zamora et al.  and Suntharalingam et al.  used MEMENTO data to model N2O production and consumption processes on global and regional scales. Freing et al.  used the database to compute global N2O production rates from the in situ measurements. A list of associated publications is available on the MEMENTO website.
MEMENTO data are freely available to interested users, who can access the database via the MEMENTO website. We would like to expand our database, so please consider adding your CH4 and N2O data. Contact us to obtain the log-in information to the database and information on how to submit your data to MEMENTO.
MEMENTO is supported by European Cooperation in Science and Technology (COST) Action 735, the Surface Ocean–Lower Atmosphere Studies (SOLAS) Project Integration Programme and the German Federal Ministry for Education Research project Surface Ocean Processes in the Anthropocene, Grant FKZ 03F0660A. The database is receiving technical support from the Kiel Data Management Team at GEOMAR Helmholtz Centre for Ocean Research.