alineación del texto en el título de la tabla

Question

Estoy intentando crear un título en la parte inferior de la tabla, pero quiero que el texto continúe debajo del número de tabla. En lugar de eso me sale esto:

y este es mi código. ¡Gracias!

\begin{table}
\centering
 \begin{threeparttable}
   
     \begin{tabular}{lllll}
        \toprule
\multicolumn{1}{l}{}    ($\delta_i,c_i$)          & \textbf{low} $\boldsymbol{c_i}$} & \textbf{high} $\boldsymbol{c_i}$}                      \\ \midrule
\multirow{4}{*}{\textbf{high} $\boldsymbol{\delta_i}$}} & EU (13.8,54.8)              & \multicolumn{1}{c}{\multirow{4}{*}{-}} \\
                                        & India (11.7, 65.8)           & \multicolumn{1}{c}{}                   \\
                                        & China (11.0, 18.8)           & \multicolumn{1}{c}{}                   \\
                                        & US (10.6, 32.9)              & \multicolumn{1}{c}{}                   \\ \midrule
\multirow{3}{*}{\textbf{low} $\boldsymbol{\delta_i}$}}  & Russia (3.5, 93.9)         & Japan (2.4, 219.2 )                            \\
                                        & Brazil (2.9, 34.6)         & Canada (1.0, 328.8)                         \\
                                        &                  & S. Africa (0.7, 328.8)                        \\ 
                                       % \cmidrule(l){2-3}  
                                       \bottomrule
     \end{tabular}
    \begin{tablenotes}
      \small
      \item Source: \citet{holtsmark2020}, based on estimates from \citet{nordhaus2015climate} and Mclkinsey (2009).  \\
 \caption{ A high $\delta_i$ implies high marginal damages from emissions while a high $c_i$ implies high marginal abatement costs (for any given level of abatement) and for our model specification $c_i=1/\phi_i$.}
      
      
       \end{tablenotes}
  \end{threeparttable}
\end{table}   

Answer 1

Tampoco estoy seguro de cómo te gusta tener subtítulos formateados. Por ejemplo como:

En este caso su MWE, usar threeparttablexy limpiar todo multicolumnlo que no sea necesario puede ser:

\documentclass{article}
\usepackage{amsmath}
\usepackage{booktabs, multirow}
\usepackage[referable]{threeparttablex}
\usepackage{caption}
\captionsetup[table]{font=small, labelfont = {bf, sf}, 
              singlelinecheck=false}

\begin{document}
    \begin{table}
    \centering
 \begin{threeparttable}
     \begin{tabular}{lll}
        \toprule
($\delta_i,c_i$) & \textbf{low} $\boldsymbol{c_i}$ & \textbf{high} $\boldsymbol{c_i}$ \\ 
        \midrule
\multirow{4}{*}{\textbf{high} $\boldsymbol{\delta_i}$} 
    & EU (13.8,54.8)        &  \multirow{4}{*}{---}     \\
    & India (11.7, 65.8)    &                           \\
    & China (11.0, 18.8)    &                           \\
    & US (10.6, 32.9)       &                           \\
        \midrule
\multirow{3}{*}{\textbf{low} $\boldsymbol{\delta_i}$} 
    & Russia (3.5, 93.9)    & Japan (2.4, 219.2 )       \\
    & Brazil (2.9, 34.6)    & Canada (1.0, 328.8)       \\
    &                       & S. Africa (0.7, 328.8)    \\
        \bottomrule
     \end{tabular}
    \begin{tablenotes}[flushleft, online]\footnotesize
    \source{Holtsmark and Weitzman (2020), based on estimates from Nordhaus(2015) and Mclkinsey (2009).}
      \end{tablenotes}
 \caption{ A high $\delta_i$ implies high marginal damages from emissions while a high $c_i$ implies high marginal abatement costs (for any given level of abatement) and for our model specification $c_i=1/\phi_i$.}
 \end{threeparttable}
    \end{table}
\end{document} 

Answer 2

No estoy seguro de haber entendido perfectamente lo que quieres, pero ¿es algo como esto?

\documentclass{article}
\usepackage{amsmath}
\usepackage{booktabs}
\usepackage{caption}
\usepackage{multirow}
\usepackage{threeparttable}

\begin{document}

\begin{table}
\centering
\captionsetup{labelfont = {bf, sf}, indention=1cm}
 \begin{threeparttable}
     \begin{tabular}{lll}
        \toprule
\multicolumn{1}{l}{} ($\delta_i,c_i$) & \textbf{low} $\boldsymbol{c_i}$ & \textbf{high} $\boldsymbol{c_i}$ \\ \midrule
\multirow{4}{*}{\textbf{high} $\boldsymbol{\delta_i}$} & EU (13.8,54.8) & \multicolumn{1}{l}{\multirow{4}{*}{---}} \\
                                        & India (11.7, 65.8) & \multicolumn{1}{c}{} \\
                                        & China (11.0, 18.8) & \multicolumn{1}{c}{} \\
                                        & US (10.6, 32.9) & \multicolumn{1}{c}{} \\ \midrule
\multirow{3}{*}{\textbf{low} $\boldsymbol{\delta_i}$} & Russia (3.5, 93.9) & Japan (2.4, 219.2 ) \\
                                        & Brazil (2.9, 34.6) & Canada (1.0, 328.8) \\
                                        & & S. Africa (0.7, 328.8) \\
                                       % \cmidrule(l){2-3}
                                       \bottomrule
     \end{tabular}
    \begin{tablenotes}[flushleft, online]
      \small
      \item[Source:] Holtsmark and Weitzman (2020), based on estimates from Nordhaus(2015) and Mclkinsey (2009).
      \end{tablenotes}
 \caption{ A high $\delta_i$ implies high marginal damages from emissions while a high $c_i$ implies high marginal abatement costs (for any given level of abatement) and for our model specification $c_i=1/\phi_i$.}
  \end{threeparttable}
\end{table}

\end{document}