def what$\_$year$\_$debug$($fraction$=0\mathrm{.}5,$ source$=$'Both'$)$:

# Calculate cumulative sums of emissions by year for each source

df$\_$emissions$[\text{'}$Cumulative$\_$Fossil'$]=$ df$\_$emissions$[[\text{'}$Coal$\text{'},$ 'Oil', 'Gas', 'Cement', 'Flaring', 'Other'$]].$sum$($axis$=1).$cumsum$()$

df$\_$emissions$[\text{'}$Cumulative$\_$Land'$]=$ df$\_$emissions$[\text{'}$Land$\text{'}].$cumsum$()$

df$\_$emissions$[\text{'}$Cumulative$\_$Both'$]=$ df$\_$emissions$[\text{'}$Cumulative$\_$Fossil'$]+$ df$\_$emissions$[\text{'}$Cumulative$\_$Land'$]$

# Get the total cumulative emissions for the specified source

total$\_$emissions $=$ df$\_$emissions$[$f$\text{'}$Cumulative$\_\{$source$\}\text{'}].$iloc$[-1]$

# Find the year where the cumulative emissions exceed the fraction of interest

target$\_$emissions $=$ total$\_$emissions $*$ fraction

year$\_$row $=$ df$\_$emissions$[$df$\_$emissions$[$f$\text{'}$Cumulative$\_\{$source$\}\text{'}]>=$ target$\_$emissions$].$iloc$[0]$

# Return the year as an integer

return int$($year$\_$row$[\text{'}$Year$\text{'}])$